What happened
Ten year UK government borrowing costs climbed above 5.20 percent this week, the highest level since 2008, while thirty year borrowing costs reached 5.85 percent, a rate last seen in 1998. Both moves happened over two trading sessions as investors returned from the late August bank holiday weekend.
A gilt is an IOU issued by the UK government. When the Treasury needs money it sells gilts to investors, promising to pay interest and hand back the original sum on a set date. The yield is the annual return an investor earns by buying that IOU at the current market price. Prices and yields move in opposite directions, so a rising yield means investors are paying less for British government debt and demanding more compensation to hold it.
The trigger was global rather than purely domestic. Long dated government bond yields rose across the United States, Japan and the euro area at the same time, in a broad move away from long term government debt. Sterling actually slipped on Tuesday even as UK yields jumped, which is telling. Normally higher yields attract foreign money and lift a currency. When they do not, it usually means investors are demanding a risk premium rather than chasing a better return.
Bloomberg reported on 1 September that the surge has halved the fiscal headroom the Treasury holds against its own borrowing rules. Headroom is the spare margin between what the government plans to borrow and the maximum its self imposed rules allow. Debt interest is already one of the single largest lines in the public finances, so when yields jump the annual interest bill rises and that margin shrinks fast.
Why it matters
Gilt yields are the foundation layer of almost every interest rate in Britain. Banks price mortgages, business loans and savings products off the cost of government borrowing, because lending to the state is treated as the safest available option. If the safest borrower in the country has to pay 5.20 percent, nobody else is going to pay less.
The most direct channel runs through swap rates, which are the wholesale rates lenders use to fix their own funding costs before they can offer you a fixed rate mortgage. Swap rates track gilt yields closely. When gilts sell off, swaps follow within days, and mortgage repricing usually follows within two to three weeks.
The second channel is fiscal. Every extra percentage point on government debt costs the Treasury billions a year in interest. That money does not go to schools, hospitals or roads. With headroom halved, the Chancellor faces a narrower set of choices at the autumn budget: raise taxes, cut planned spending, or loosen the borrowing rules and risk a further market reaction.
Third, there is the effect on anyone holding bonds. Pension funds, insurers and the bond portion of workplace pension defaults all own gilts. When yields rise, the market value of existing bonds falls, because older bonds paying lower interest look less attractive next to newly issued ones.
Explained simply
Think of a gilt yield as the interest rate on the national overdraft. This week the market quietly decided Britain is a slightly riskier customer, and moved the rate up accordingly.
Imagine the government as a household that spends more than it earns and covers the gap by borrowing. It goes to a room full of lenders and asks who will lend it money for ten years. The lenders bid. If they are relaxed, they accept a low rate. If they are nervous about inflation, about how much the household already owes, or about better offers elsewhere, they demand a higher one.
What happened this week is that the lenders got nervous everywhere at once. American, Japanese and European borrowers all had to pay up. Britain paid up more than most, because it borrows a lot, because a large slice of its debt is linked to inflation, and because a budget is coming in which the spending plans could change.
Now follow the chain. The government pays more, so the interest bill grows. The interest bill grows, so there is less money for everything else. Meanwhile the banks, which price their own products off that same government rate, quietly raise the cost of your next mortgage. None of it is announced. It simply shows up in the numbers a few weeks later.
What it means for you
If you are remortgaging. The average two year fixed mortgage sat at 5.52 percent and the average five year fix at 5.64 percent as of 1 September. Lenders including Nationwide and Santander had been trimming rates in recent weeks. If gilt and swap rates stay at these levels, those cuts are likely to stall and could partly reverse. If your fix ends within the next six months, it is worth securing a rate now, since most offers can be held for three to six months and swapped later if pricing improves.
If you are saving. Fixed rate savings bonds and fixed rate cash ISAs are priced off the same swap curve, so higher gilt yields tend to support them. One year fixed accounts are the ones most likely to hold up. Easy access rates, by contrast, track the Bank Rate of 3.75 percent much more closely and will not benefit in the same way.
If you are approaching retirement. Higher gilt yields are genuinely good news if you plan to buy an annuity, the product that converts a pension pot into a guaranteed income for life. Annuity rates are built almost directly on long dated gilt yields, so a thirty year yield near 5.85 percent means a materially higher income for the same pot than a year ago.
If you hold bond funds. Gilt funds and the bond portion of lifestyled workplace pensions will have fallen in value this week. This is the mirror image of the annuity point, and it matters most if you are within ten years of retirement and your pension has been automatically shifted into bonds. It is worth checking what your default fund actually holds.
The bigger picture
The obvious comparison is the autumn of 2022, when a package of unfunded tax cuts sent gilt yields sharply higher and forced emergency intervention. This episode is different in character. It is slower, it is global, and it has not been driven by a single domestic announcement. But the destination looks similar: a government paying far more to borrow than it assumed when it drew up its plans.
The next scheduled marker is the Bank of England decision on 17 September, with Bank Rate currently at 3.75 percent. Markets put the probability of no change at roughly 86 percent. The Bank sets short term rates, though, and this selloff is concentrated at the long end, so a hold would do little to bring thirty year yields down.
Watch three things over the coming weeks. First, whether swap rates follow gilts higher, which would confirm mortgage repricing is coming. Second, the next inflation reading, since long dated yields are essentially a bet on inflation over decades. Third, any signal on how the autumn budget will close the gap that these borrowing costs have opened up.
