Index-Linked Gilts and TIPS: A Guide to Inflation-Linked Bonds for UK Investors

Inflation is one of the most insidious threats to long-term purchasing power. A 3% annual inflation rate halves the real value of a portfolio in approximately 24 years, even if nominal values are preserved. Inflation-linked bonds — securities whose principal and interest payments adjust with inflation — offer investors direct protection against this risk. For UK investors, the primary instruments are index-linked gilts and US Treasury Inflation-Protected Securities (TIPS).

This guide explains how these instruments work, the important differences between UK and US instruments, the real yield concept, the significant duration risk that many investors underestimate, and the circumstances in which inflation-linked bonds are most useful.

Important: Index-linked bonds carry significant interest rate risk, particularly duration risk, and can lose substantial value if real interest rates rise. Their complexity means they are not always suitable as simple "inflation protection." The value of investments can fall as well as rise. This guide is for information only and does not constitute financial advice.

How Index-Linked Bonds Work

Conventional bonds pay a fixed nominal coupon and return the face value at maturity. An index-linked bond adjusts both the principal (and therefore the coupon, which is a fixed percentage of the adjusted principal) in line with an inflation index.

UK index-linked gilts are linked to the UK Retail Price Index (RPI), measured with an eight-month lag. So a gilt purchased today incorporates inflation data from eight months ago. At maturity, the investor receives the original principal multiplied by the cumulative RPI increase over the gilt's life. If RPI has risen 50% over 20 years, the investor receives 150% of the original principal plus inflation-adjusted coupons throughout.

US TIPS use the same mechanics but link to the Consumer Price Index (CPI-U) with a three-month lag. A key difference from UK gilts is that TIPS have a "floor" — the adjusted principal cannot fall below the original face value at maturity (i.e., in a deflationary scenario, investors receive at minimum the original principal). UK index-linked gilts do not have a formal floor at maturity, though deflation has not been a significant practical issue historically.

RPI vs. CPI: A Significant UK-Specific Issue

One of the most important — and frequently misunderstood — aspects of UK inflation-linked gilts is that they are linked to RPI, not CPI. This matters enormously because RPI and CPI measure inflation differently and have historically diverged by 0.5–1.0 percentage points per year, with RPI almost always higher.

Why RPI is higher than CPI:

• RPI includes housing costs (mortgage interest payments and council tax) that CPI excludes
• RPI uses an arithmetic mean formula while CPI uses a geometric mean — the formula difference alone accounts for approximately 0.5% of the RPI-CPI gap (the "formula effect")

Policy implications: The UK government recognises that RPI is a flawed measure and the Office for National Statistics no longer recommends it as a general measure of inflation. In 2020, the UK government announced that RPI would be aligned with CPIH (CPI including owner-occupied housing costs) from 2030 — a decision with massive implications for index-linked gilt holders. CPIH runs approximately 1% per annum lower than current RPI, meaning the switch will significantly reduce the inflation compensation received by holders of long-dated index-linked gilts that mature after 2030.

This reform has been a source of significant legal challenge from pension funds and institutional investors who hold long-dated index-linked gilts and argue they are receiving less than originally contracted. For individual investors, it means that index-linked gilts are less effective as long-term inflation protection than they might appear — particularly for long-dated instruments approaching the 2030 transition.

The Real Yield Concept

A nominal gilt yielding 4.5% provides a gross nominal return of 4.5%. If inflation is 2.5%, the real (inflation-adjusted) return is approximately 2% (in practice, using the Fisher equation: real yield ≈ nominal yield – inflation).

Index-linked gilts and TIPS are quoted in terms of real yield — the yield after adjusting for expected inflation. A UK index-linked gilt with a real yield of 0.5% means the investor will earn 0.5% above RPI annually, provided the gilt is held to maturity.

Real yields can be negative: during the 2010s and early COVID period, UK index-linked gilt real yields fell well below zero — investors were accepting a real return below inflation in exchange for the certainty of inflation protection and the safety of government credit. As central banks raised nominal rates from 2022 onwards, real yields rose significantly — UK 10-year index-linked gilt real yields moved from roughly -3% to approximately +0.5–1.0% by 2024–2026. This rise in real yields was the single biggest driver of significant losses for holders of long-dated index-linked gilts.

Duration Risk: The Most Important Warning

Perhaps the most commonly misunderstood aspect of index-linked bonds is their duration. Duration measures the sensitivity of a bond's price to changes in yields — specifically, a duration of 20 means that a 1% rise in real yields will cause approximately a 20% fall in the bond's price.

UK index-linked gilts are famous for their extreme duration — many outstanding index-linked gilts have maturities of 30–50 years, and because their coupons are very low (typically 0.125–1.25%), their effective duration is among the highest of any government bond globally. Some long-dated UK index-linked gilts have modified durations in excess of 30.

This means that if real interest rates rise by 1%, these bonds can lose 30% or more of their value. This is precisely what happened in 2022: the sharp rise in real yields caused catastrophic losses for portfolios concentrated in long-dated UK index-linked gilts — most infamously the liability-driven investment (LDI) strategies used by defined benefit pension funds, which triggered the September 2022 mini-budget crisis and the Bank of England's emergency gilts purchase programme.

The practical implication for investors: Index-linked bonds are not simply "safe" inflation protection. They are interest rate-sensitive instruments, and their duration risk can dwarf the inflation protection they provide in the short to medium term. An investor buying a 30-year index-linked gilt at a real yield of -2% who then experiences real yields rising to +0.5% would suffer a capital loss far in excess of any inflation protection benefit over a typical investment horizon.

Shorter-dated index-linked gilts and TIPS have lower duration — a five-year TIPS has a modified duration of approximately 4–5 — and carry proportionately lower interest rate risk.

Breakeven Inflation Rate

The breakeven inflation rate is the key concept for deciding between nominal and index-linked bonds.

Breakeven = nominal yield – real yield

If a conventional 10-year gilt yields 4.5% and a 10-year index-linked gilt yields 0.5% (real), the 10-year breakeven is 4.0%. This means:

• If actual RPI (or CPI for TIPS) averages above 4.0% over the next 10 years, the index-linked gilt will outperform the nominal gilt
• If actual inflation averages below 4.0%, the nominal gilt will outperform

The breakeven rate is, in effect, the market's consensus expectation of average inflation over the bond's life. Investors who expect inflation to be persistently above the breakeven will prefer index-linked; those who expect inflation to fall below the breakeven will prefer nominal.

Breakeven rates move continuously with market conditions and provide a real-time read on inflation expectations. When breakeven rates fall (markets expecting lower inflation), index-linked bonds tend to underperform; when breakeven rates rise, they outperform.

US TIPS: An Alternative for UK Investors

US Treasury Inflation-Protected Securities (TIPS) offer similar inflation protection to UK index-linked gilts but with several differences:

CPI linkage: TIPS link to US CPI-U, not UK RPI. For UK investors, this creates a basis risk — US inflation may not match UK inflation, particularly if sterling/dollar exchange rates change or if UK and US monetary policies diverge.

Three-month lag (shorter than the eight-month UK lag): TIPS respond slightly faster to current inflation data.

Deflation floor: As noted above, TIPS guarantee investors at least the original principal at maturity, even if US CPI turns negative.

Liquidity: The TIPS market is extremely liquid — one of the largest and most actively traded inflation-linked bond markets in the world.

Currency risk: UK investors in USD-denominated TIPS are exposed to GBP/USD exchange rate movements. This can amplify or offset inflation protection depending on currency movements.

Inflation-Linked Bond ETFs

For most investors, ETFs provide more practical access to inflation-linked bonds than direct gilt purchases:

UK index-linked gilts:

• iShares £ Index-Linked Gilts ETF (INXG): The largest UK index-linked gilt ETF; tracks across various maturities; significant long-duration exposure
• Lyxor UK Gilts Inflation Linked ETF: Alternative access to UK index-linked gilts
• iShares 0-5 Year £ Index-Linked Gilts ETF (INXG 0-5Y variant): Shorter-duration option to reduce interest rate risk

US TIPS:

• iShares $ TIPS ETF (ITPS): USD-denominated; GBP currency risk applies
• Vanguard Short-Term Inflation-Protected Securities ETF: Short-duration TIPS; lower interest rate risk than broad TIPS ETFs
• iShares Global Inflation Linked Government Bond ETF (GIGU): Blends UK index-linked gilts, TIPS, and other global inflation-linked bonds; provides diversification across inflation measures

When to Add Inflation-Linked Bonds to a Portfolio

Index-linked bonds and TIPS are most valuable in a portfolio under specific conditions:

When breakeven inflation rates are low: If the market expects low future inflation, index-linked bonds are cheaply priced relative to their protection value. Buying inflation protection when no one is worried about inflation is the most efficient time to do so.

When real yields are positive: Following the 2022–2024 normalisation, real yields on UK and US inflation-linked bonds are meaningfully positive — offering a genuine real return above inflation if held to maturity. This is substantially more attractive than the negative real yields of 2015–2021.

For investors with inflation-linked liabilities: Defined benefit pension funds have long been the natural buyers of index-linked gilts, as their liabilities (pension payments) are often inflation-linked. For individual investors with similar characteristics — for example, those with long-term spending commitments that rise with inflation — index-linked bonds provide a genuine liability-matching function.

As a portfolio diversifier: Inflation-linked bonds have historically been positively correlated with equities in inflationary bear markets (where equities underperform due to rising discount rates, but inflation-linked bonds provide compensation through their index linkage) — though 2022 showed that when real rates rise rapidly, even inflation-linked bonds can suffer significant drawdowns alongside equities.

Not as a short-term trading instrument: The combination of low coupons, high duration, and the eight-month lag in UK gilts makes these poor instruments for short-term positioning.

How Global Investments Can Help

Inflation-linked bonds occupy an important but nuanced role in long-term portfolio construction. Their seemingly straightforward inflation protection conceals meaningful duration risk, basis risk (particularly for TIPS held by UK investors), and — in the UK context — regulatory risk around the RPI-to-CPIH transition. Getting the allocation right requires understanding the relationship between real yields, breakeven rates, and portfolio duration.

At Global Investments, we help clients determine whether and how to include inflation-linked bonds — balancing the genuine protection benefits against duration and basis risks, and selecting appropriate maturities and instruments for each client's time horizon and liability profile. Contact our team to discuss fixed income allocation within your portfolio.

This guide is for information purposes only and does not constitute financial advice. Index-linked bonds and TIPS carry significant interest rate risk and can lose substantial value if real yields rise. Currency risk applies to non-sterling denominated instruments. The value of investments can fall as well as rise. Always seek qualified professional advice before making investment decisions.