The shadow chancellor’s call on the eve of the Budget comes after demands from charities and cross-party MPs, including more than a dozen Tories, for Philip Hammond to commit extra money towards the government’s flagship welfare programme.
Mr McDonnell also accused the government of showing “callous complacency” over universal credit after repeated suggestions that low-income families are being driven into debt.
“I’m saying to other political parties that if he doesn’t halt the rollout of universal credit we’ve got to vote this Budget down,” he told Sky News’ Sophy Ridge on Sunday. “We’ve got to stop him forcing people into poverty in this way.”
Mr McDonnell’s intervention came as Mr Hammond also hinted Monday’s Budget would contain extra cash for universal credit, and admitted there had been “teething issues” with the new system.
“My aspiration would be to try to smooth this process,” he said on the BBC’s Andrew Marr show. “Where we see issues that need addressing we will address them.”
Mr Hammond’s colleague Liz Truss, chief secretary to the Treasury, also echoed his comments, and said the Treasury is “listening very closely to what our colleagues have to say”, adding: “We want this system to work.”
But asked whether people would be worse off under universal credit – a claim made by the cabinet minister, Esther McVey, in charge of the rollout – the chancellor replied: “I hope not.”
Rebels in the Conservatives, including Iain Duncan Smith, one of the architects of the policy, have previously said the Treasury should reverse the cuts made by his predecessor George Osborne and commit £2bn to protect the hardest hit groups.
The Commons Work and Pensions Committee has also said the government’s plan to put 4 million more people on the benefit must be halted without changes to prevent “unmanageable debt”.
In a highly critical report – published on Sunday – MPs on the committee warned that the support offered to claimants transferring from the legacy benefits system to universal credit was “woefully inadequate” and risked undermining the whole project.
Speaking earlier this month, John Major, the former prime minister, warned Theresa May she could face a backlash over universal credit similar to that experienced by Margaret Thatcher with the doomed poll tax.
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