The UK’s shaky bond market is at the heart of investor worries that the war in Iran is about to unleash faster inflation and strain government finances.As yields soared globally on concern central banks will be forced to tighten monetary policy, the UK endured some of the most extreme market moves. The 10-year yield, a benchmark for Britain’s borrowing costs, shot to 5% for the first time since the 2008 financial crisis.It’s a grim milestone for a country already contending with fragile finances while Prime Minister Keir Starmer faces threats of leadership challenges from within the governing Labour Party. Scars from the ill-fated mini-budget of 2022, along with signals that the Bank of England (BoE) is ready to raise interest rates, have put the UK in the “penalty box,” said James Athey of Marlborough Investment Management.“It’s been an intense week,” he said. “As seems all too familiar, gilts have fared worse than most.”Ten-year gilt yields jumped 16 basis points on Friday. Moves in short-term notes have been even more pronounced. In the span of two days, the two-year yield has increased by almost 50 basis points, or half a percentage point. The two-year yield was up 19 basis points to 4.59% in Friday’s session.For a sense of how quickly things have turned, it was just three weeks ago that confidence was running high that the BoE would cut rates. Now, money markets are betting on three quarter-point hikes this year. Odds of a fourth are at a coin toss.The reason the UK is so vulnerable is its reliance on imported energy, track record for sticky inflation and dependence on foreign borrowing — all of which have left the country more exposed to the fast-changing whims of bond traders.The moves have also spilled over to other markets including US Treasuries, where yields were higher across maturities.“In an alternative reality, where the conflict in the Middle East never happened, the Bank of England would have just cut interest rates and gilts would be heading for 4%, spurred on by weaker wage data,” said Matthew Amis, an investment director at Aberdeen. “That shows the vulnerability of the UK’s economic position today.”For the UK government, the jump in yields is a bigger burden than it was back in 2008, when the country’s debt to as a proportion of overall growth was half the current level. To make matters worse, data released Friday showed that borrowing last month came in above forecasts at £14.3bn.The recent increase in borrowing costs alone would shave £3bn off of Chancellor of the Exchequer Rachel Reeves’ £23.6bn buffer against breaching her main fiscal rule, according to a Bloomberg Economics analysis published on Friday. That doesn’t account for inflation, slower growth and any new spending to reduce energy costs for households, something a Sky News/YouGov poll released earlier this week found 84% of Britons support.Reeves has argued the UK’s finances and energy mix are in a better position to absorb energy shocks, she has balked at backing a costly support package like the one implemented after the last price surge in 2022. She told the Times newspaper last week that the government would give “targeted” support “to those who really need it.”Earlier in the week, Culture Secretary Lisa Nandy had raised the prospect of loosening the government’s limits on borrowing during a Cabinet meeting. Nandy, who is seen as a voice among the Labour Party’s “soft left,” suggested the rules might need to be relaxed if prices continued to rise and a major support package was necessary, according to a person familiar with the discussions.Ministers have made similar suggestions before the current crisis and the government isn’t considering any change, according to the person, who asked not to be identified discussing confidential meetings. Nandy, whose intervention was first reported by the Times newspaper late Thursday, didn’t immediately respond to a request for comment.“The chancellor’s fiscal rules are non-negotiable,” Starmer’s spokesman Tom Wells told reporters. “They ensure we’re getting borrowing and debt down.”