Living standards for all UK families are set to fall by 2030, with those on the lowest incomes declining twice as fast as middle and high earners, according to new data that raises serious questions about Keir Starmer’s pledge to make working people better off.

The grim economic analysis, produced by the respected Joseph Rowntree Foundation (JRF), comes before the chancellor, Rachel Reeves, makes her spring statement on Wednesday in which she will announce new cuts to public spending rather than increase borrowing or raise taxes, so as to keep within the government’s “iron clad” fiscal rules.

In December, the prime minister announced a series of new ­“milestones” that he said would be passed before the next general ­election, which is likely to be held in 2029. The first of these was “putting more money in the pockets of ­working people”.

But with many Labour MPs already deeply concerned over Reeves’s plan to raise about £5bn by cutting ­benefits, including for disabled ­people, evidence that living standards are on course to fall markedly under a Labour government – and to decline most for the least well off – will add to the mood of growing disquiet in party’s ranks.

The JRF analysis rests on a ­realistic assumption that the Office for Budget Responsibility (OBR) will adjust its forecasts in line with the Bank of England and other main forecasters when it makes them public on Wednesday. The OBR is expected to halve the expected growth rate for this year from 2% to about 1%.

In what it describes as a “dismal reality”, the JRF said its detailed analysis shows that the past year could mark a high point for living standards in this parliament. It concludes that the average family will be £1,400 worse off by 2030, representing a 3% fall in their disposable incomes. The lowest income families will be £900 a year worse off, amounting to a 6% fall in the amount they have to spend.

The JRF also said that if living standards have not recovered by 2030, Starmer will not only have failed to pass his No 1 milestone but will also have presided over the first government since 1955 to have seen a fall in living standards across a full parliament.

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Alfie Stirling, director of insight and policy at JRF, said further cuts were not the way to reverse the trend of falling living standards. Instead, he argued, Reeves should consider raising tax for the wealthiest.

“There is no doubt the government is facing an unenviable list of economic pressures and uncertainties, ranging from the domestic to the international. But how you manage these risks is a matter of political choice…

“It is wrong, and ultimately counterproductive, to try and rebuild the public finances through cuts to disability benefits. Instead, government should be addressing hardship and raising living standards directly, as part of their strategy for growth.

“Fiscal pressures should be met through tax reform. There are a number of options to raise revenue from those with the broadest shoulders, while also supporting growth by removing perverse incentives in the tax system and staying within the government’s manifesto commitments.”

Earlier last week a group of leading economists wrote to the Financial Times warning that it would be a “profound mistake” for ministers to cut spending or investment, adding that “the UK cannot cut its way to growth”.