Britain’s inflation rate rose to a 40-year high of 10.1 per cent in September, as prime minister Liz Truss committed to increasing state pensions in line with prices next year.

The rate of inflation according to the consumer price index rose from 9.9 per cent in August, driven by the highest food price increases in decades, and exceeded economists’ expectations.

The figures, released on Wednesday, come as Truss’s government is trying to find £40bn of savings to put public finances on a sustainable footing following its failed “mini” Budget of last month.

Early in the day Jeremy Hunt, Britain’s new chancellor, declined to confirm that the government would honour its manifesto promise to keep the “triple lock” through which pensions are increased each year by the whatever is highest of inflation, earnings growth or 2.5 per cent.

But speaking at prime minister’s question time at the House of Commons, the under fire Truss reaffirmed the government’s commitment to raising pensions in line with inflation and “protecting the triple lock on pensions”.

The government’s standard practice is to use the September inflation figure to determine the increase in pensions and benefits the following April.

Truss did not make any similar commitment to non-pensioner benefits.

George Dibb, head of the Centre for Economic Justice at the IPPR think-tank, said the “steeper rise in essentials such as food and drink where prices are now rising at over 14 per cent . . . underlines the need for greater support for the most vulnerable households this winter over and above the energy price cap”.

The details of the figures showed there was an increase in the core index as well as higher food prices.

The core rate, excluding energy, food, alcohol and tobacco, rose from an annual rate of 6.3 per cent in August to 6.5 per cent in September.

The Office for National Statistics said the overall consumer price index rose 0.5 per cent in September compared with August, a larger increase over the month than in 2021 when the index rose only 0.3 per cent.

At more than five times the Bank of England’s 2 per cent target, the double-digit inflation rate will also add to pressure on the central bank for a large interest rate rise on November 3.

The BoE will need to weigh the additional price pressures against the government’s U-turns on unfunded tax cuts and less generous relief on household energy costs, which will reduce medium-term pressures on prices.

Samuel Tombs, chief UK economist at Pantheon Macroeconomics, said the “MPC still is a long way from being able to claim victory” over inflation, but urged the central bank to worry more about “weakening consumer demand and emerging slack in the labour market” than the current high level of inflation.

Paul Dales, chief UK economist at Capital Economics, said the rate of inflation would rise to 10.5 per cent in October and to 11 per cent in April once the government’s energy price guarantee expired.

“Today’s release highlights the danger that underlying inflation remains strong even as the economy weakens,” he said.