Labour leadership candidate says he will abstain in Monday’s vote despite previously arguing party should vote against bill

Andy Burnham has said he will not vote against the government’s welfare bill, despite previously describing it as “unsupportable”.

In a letter to Labour MPs, the leadership hopeful said he would toe the party line and abstain in Monday night’s critical vote because collective responsibility was important and was what he would expect from his MPs if he was their leader.

Labour’s acting leader, Harriet Harman, provoked anger within the party when she announced that her MPs would be abstaining on the government’s welfare bill, supporting the introduction of a welfare cap and the restriction of tax credits to a claimant’s first two children.

Harman tried to defuse a rebellion over the decision by tabling an amendment, which is almost certain to be defeated, setting out why the party disagrees with the government’s proposed bill.

Burnham, Yvette Cooper and Jeremy Corbyn – three of the four leadership candidates – publicly opposed Harman’s position, with Burnham saying the party should vote against the bill if the government did not accept a raft of Labour amendments.

Despite disagreeing with Harman’s position, Cooper has been clear that she will abstain on the bill, as will Liz Kendall, who supported the acting leader’s decision. Jeremy Corbyn will vote against.

In his letter, Burnham admitted that he had led calls for the party to change its position, but sought to reassure Labour MPs that it was “only the beginning of a major fight with the Tories”.

“I am determined that we will fight this regressive bill line by line, word by word in committee,” he added.

Burnham said that as leader he would oppose the proposed limit on child tax credits to two children per family and that he would vote against the bill at third reading if the government did not make major changes to it during committee stage.

“As you know, I was very clear last weekend that we could not simply abstain on this bill and that we needed to set out where we have agreement with reforms, but more importantly, where we strongly disagree,” he writes in the letter.

“Our reasoned amendment sets out clearly our opposition to many aspects of the bill. In truth, it could be stronger but it declines to give the bill a second reading and, therefore, voting for it tonight is the right thing to do.”

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Burnham echoed Harman’s reasons for asking Labour MPs to abstain on the bill, saying: “The Tories want to use this period to brand us in the way they did in 2010. We must not allow that to happen.”

Harman argued that the party could not simply tell the public they were wrong after two general election defeats in a row, adding it had been defeated because it had not been trusted on the economy or benefits.

Writing in the Guardian, George Osborne urged progressive MPs in the Labour party to back his welfare changes in the vote, saying they should recognise that the proposals not only chime with the public but build on mainstream Labour thinking.

The chancellor called on Labour to stop blaming the public for its defeat and recognise that welfare requires public consent.