Lower-income households are on track to become £500 a year poorer by the end of the decade as a result of the UK chancellor’s spring statement, according to analysis by a leading thinktank.

The Resolution Foundation said a combination of weak economic growth over the next five years and benefit cuts that fell disproportionately on lower-income households would result in an average annual loss of £500 in 2030 for those in the poorest half of the population.

Rachel Reeves has been criticised by unions and political opponents after she made £14bn of savings in her spring statement on Wednesday to keep the public finances on track.

The Resolution Foundation’s figure contrasts with the chancellor’s claim in her address that people would be “on average over £500 a year better off” under the Labour government.

Reeves restored a surplus of £9.9bn in the government’s day-to-day spending to meet a budget rule forcing her to balance income and expenditure by the end of the parliament.

The chancellor was responding to a report by the Treasury’s independent forecaster, the Office for Budget Responsibility (OBR) that said higher debt interest payments and weaker economic growth would reduce tax receipts and push up Whitehall spending during the parliament.

An estimated 250,000 more people, including 50,000 children, will be living in relative poverty after housing costs by the end of the decade as a result of the government’s squeeze on welfare, according to an impact assessment by the department of work and pensions.

The Resolution Foundation said about 3 million households on incapacity benefits would be affected. About 800,000 claimants will have reduced personal independence payments, saving the government £8.1bn by 2029-30.

Ruth Curtice, the Resolution Foundation’s chief executive, said: “The outlook for living standards remains bleak. Britain’s poor economic performance, combined with policies that bear down hardest on those on modest incomes, mean that 10m working-age households across the bottom half of the income distribution are on track to get £500 a year poorer over the course of the parliament.”

On Thursday morning, Reeves appeared to reject the official assessment of her changes to welfare and said she was “absolutely certain” people would not be pushed into poverty.

She told Sky News: “I am absolutely certain that our reforms, instead of pushing people into poverty, are going to get people into work. And we know that if you move from welfare into work, you are much less likely to be in poverty.

“That is our ambition: making people better off, not making people worse off, and also the welfare state will always be there for people who genuinely need it.”

Economists warned of further uncertainty before the autumn budget, while opposition critics accused Reeves of mismanaging the public finances, and unions said the policy changes marked a return to austerity.

James Smith, a research director at the Resolution Foundation, said: “The 2020s are looking like being a disaster for living standards, even compared to the previous decade.”

He said welfare cuts were rushed and look more like a sticking plaster to cover a hole in the public finances than a well-thought-through and planned reform of the welfare system.

The National Institute of Economic and Social Research thinktank said Reeves’s decision to adopt strict budget rules meant she needed to tinker with spending every six months, harming the ability of the public sector to make long-term plans.

The OBR halved its forecast for growth in gross domestic product in 2025 from 2% to 1%, but upgraded its forecasts for subsequent years.

The UK economy is expected to grow by 1.9% in 2026, 1.8% in 2027, 1.7% in 2028 and 1.8% in 2029, although the watchdog said that tariffs threatened by Donald Trump could wipe out the chancellor’s relatively thin £9.9bn buffer.

The US president announced on Wednesday night he was placing a 25% levy on car imports to the US, creating further uncertainty over whether the UK will be able to secure a carve-out amid an escalating trade war.

“We are not at the moment in a position where we want to do anything to escalate these trade wars,” Reeves told Sky when asked if Britain would impose retaliatory tariffs against the US. “Trade wars are no good for anyone.”