UK Index-Linked Gilts
UK Index-Linked Gilts
UK index-linked gilts adjust principal and coupons with the Retail Price Index, delivering inflation protection with a longer adjustment lag than TIPS and strong appeal to inflation-matched liabilities.
Key takeaways
- UK index-linked gilts link to the Retail Price Index (RPI), which runs 0.5–1% higher than CPI and includes housing costs
- Principal and coupon adjustments are made in arrears, meaning the current month's inflation is incorporated at the end of that month, with a final "true-up" at maturity
- An eight-month lag exists before published RPI inflation is incorporated into gilt prices, longer than the US TIPS three-month lag
- UK pension funds hold substantial index-linked gilts to match inflation-linked pension liabilities
- Real yields on UK index-linked gilts are often lower (more negative) than TIPS due to strong pension-fund demand and lower RPI expectations
RPI: a different inflation measure
The Retail Price Index (RPI) has been calculated by the UK Office for National Statistics since 1947. While CPI focuses on consumer expenditure across all households, RPI emphasizes the cost of living for a typical household and includes mortgage interest payments. This makes RPI typically 0.5–1% higher than CPI annually.
The gap widens in rising-rate environments. When central banks raise rates sharply (as in 2022–2023), mortgage costs spike, and RPI accelerates faster than CPI. An investor holding UK index-linked gilts benefits from this extra inflation capture. But when rates fall and mortgages become cheaper, RPI lags behind and gilts underperform.
Additionally, the UK moved from RPI to a modified version in 2024, addressing long-standing criticism that RPI overstated inflation. However, existing index-linked gilts remain linked to the legacy RPI formula, preserving their inflation premium. Newly issued UK inflation-linked bonds might use CPI instead, creating a split in the market.
In-arrears adjustment mechanics
Unlike US TIPS, which adjust principal monthly using a three-month lag, UK index-linked gilts adjust in arrears. The principal is adjusted at the coupon payment date to reflect the inflation that occurred in the prior eight months (measured from the publication lag of RPI data).
For example, a gilt maturing in 2030 with a 2% coupon pays interest semi-annually, typically in January and July. At the July coupon date, the principal is adjusted upward by the RPI inflation from the previous eight months, based on data available at the time. The coupon is then calculated on this adjusted principal and paid immediately.
This in-arrears approach means the final payment at maturity includes a "true-up"—the cumulative RPI inflation over the entire holding period is reflected in the principal redeemed. If you hold a gilt to maturity, you receive the original principal multiplied by (1 + total RPI inflation over the holding period), plus coupons on the progressively adjusted principal. The longer holding period (compared to TIPS) means the lag is less painful for buy-and-hold investors.
The eight-month lag and secondary market implications
The eight-month lag between inflation occurring and being reflected in gilt adjustments is substantial. When the Office for National Statistics publishes RPI data (roughly one month after the month ends), UK gilts don't immediately adjust. Instead, the Treasury waits until the coupon payment date to incorporate accumulated RPI into the principal adjustment.
This creates a distinct secondary market dynamic compared to TIPS. A UK investor seeing hot RPI inflation news must wait months for the actual principal adjustment to appear in their gilt holdings. The price of a gilt on the secondary market might move immediately to reflect new inflation expectations, but the coupon payment itself lags far behind.
In 2022, when UK inflation hit 11% (driven partly by the RPI mortgage component), index-linked gilt holders benefited from the actual principal adjustments being delayed relative to the market repricing. Prices jumped as real yields fell, but the actual cash adjustments came later. By the time coupons reflected the full inflation, some of the repricing had reversed.
Demand from pension funds
UK pension funds collectively hold tens of billions of pounds in index-linked gilts. These funds pay inflation-linked pensions to retirees, meaning their liabilities grow with RPI annually. An index-linked gilt is a perfect hedge: it pays coupons and principal that grow with RPI, matching the liability growth. For liability-matching, the real yield and coupon level matter less than the perfect correlation with the fund's outflows.
This deep, steady demand from pension funds keeps real yields on UK index-linked gilts lower (more negative) than rational expectations about future inflation might justify. In 2023, real yields on some UK index-linked gilts dipped below negative 2%, meaning buyers accepted losing 2% annually in real terms for the privilege of matching inflation-linked liabilities.
Retail investors lacking inflation-linked liabilities should think carefully before overweighting UK index-linked gilts. The low real yields reflect specialized demand from pension funds, not necessarily attractive returns for long-term wealth building.
Comparing TIPS and UK index-linked gilts
A US-based investor comparing a 10-year TIPS with a 10-year UK index-linked gilt faces several trade-offs. TIPS link to CPI (lower inflation) and adjust monthly with a three-month lag (faster adjustment). UK gilts link to RPI (higher inflation) and adjust semi-annually in arrears with an eight-month lag (slower). The TIPS are dollar-denominated (currency risk for a sterling-based investor) while the UK gilt is pound-denominated (currency risk for a dollar-based investor).
Over a 10-year holding period, the TIPS real yield might be 1.5% while the UK gilt real yield is 0.5%, a 1% gap. That gap reflects both the higher RPI inflation expectation and the pension-fund demand effect. If you believe RPI will surprise to the upside (driven by wage inflation or housing cost acceleration), the UK gilt is the better bet. If you expect RPI and CPI to move together and prefer the faster adjustment mechanism, TIPS are preferable.
Currency considerations dominate for international investors. A US investor holding UK gilts faces sterling depreciation or appreciation against the dollar. If sterling falls 10%, the sterling-denominated principal declines in dollar terms despite inflation adjustments. Hedging this currency risk typically costs 1–2% annually, eating most of the real yield advantage.
The true-up at maturity
A key feature of UK index-linked gilts is the final true-up at redemption. If you hold a gilt to maturity, the final principal payment includes all accumulated RPI inflation over the entire holding period, regardless of how the monthly adjustments were made.
An investor buying a 10-year index-linked gilt today and holding to maturity in 2034 will receive principal adjusted for total RPI inflation from 2024 to 2034. If RPI averages 3% annually, the principal is multiplied by approximately 1.344 (the compounding of 3% over 10 years). If RPI averages 1%, the principal is multiplied by 1.105. The true-up ensures zero ambiguity about the final inflation adjustment—you receive what the cumulative RPI earned, period.
This design is particularly valuable for long-term investors with extended time horizons. The lag periods and monthly volatility matter less when the path to maturity is clearly defined and the final outcome is a guaranteed real return.
Minimum price floor and deflation protection
UK index-linked gilts don't explicitly state a deflation floor at the coupon level like TIPS do, but the redemption price has a de facto floor: it is the greater of the inflation-adjusted principal or par. In severe deflation, you receive par, not a lower amount.
This floor is less explicit than the TIPS design—UK gilts holders are advised to check the prospectus for their specific holding—but it is a standard feature of modern gilt issues. Historically, gilts issued in the 1980s and 1990s sometimes lacked explicit floors, leaving holders exposed to deflation risk. Current issues have floors, protecting investors.
Real yields and breakeven inflation rates
The spread between a nominal 10-year UK government bond yield and a 10-year index-linked gilt real yield is the breakeven inflation rate (BIR). If a nominal gilt yields 4% and an index-linked gilt yields 1.5% real, the market is pricing 2.5% RPI inflation over 10 years.
This BIR is useful for calibrating inflation views. If you expect RPI to exceed 2.5%, index-linked gilts offer the better bet. If you expect it to stay below 2.5%, nominal gilts are more attractive. The BIR changes daily based on new data and shifts in inflation expectations.
In early 2024, UK gilt BIRs sat near 2%, reflecting expectations that RPI would moderate from the 2022 peak. Investors comfortable with that expectation could justify allocating to nominal gilts for higher nominal yields. Those fearing wage-driven inflation or further asset-price inflation preferred index-linked gilts despite the low real yields.
Next
Having examined the two largest inflation-linked bond markets (US TIPS and UK gilts), the Eurozone's smaller but important inflation-linked bond market comes into focus. French OAT-i bonds and other Eurozone linkers use the Harmonized Index of Consumer Prices (HICP) and have developed alongside ECB policy constraints that limit the inflation outlook for eurozone member states.