This has been the PM's most damaging U-turn yet - but the bigger cost is the political one

It has been a painful week to watch.

A U-turn in slow motion, culminating in a midnight climbdown as Number 10 agreed to concede to defiant MPs on Thursday night.

The concessions are considerable. They mean, among other compromises, that existing claimants of personal independence payments (PIP) and the health aspect of Universal Credit will be protected from welfare reforms.

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Some MPs, like Diane Abbott and Nadia Whittome, remain unconvinced, but they were never high on the list of rebels the government expected to persuade.

Ministers now hope that with the backing of MPs like Dame Meg Hillier, the chair of the Treasury Select Committee, the bill will pass the Commons.

Their problems won't end there, though.

Firstly, there is the question of money. The Resolution Foundation estimates the concessions will cost £3bn of the £5bn the chancellor hoped to save from the welfare reforms.

The prime minister's spokesperson says the changes will be fully funded in the budget and there will be no permanent increase in borrowing. They won't comment on any potential tax rises to plug the gap in Rachel Reeves's finances.

The bigger cost, though, is the political one.

A year ago, when Sir Keir Starmer strode into Downing Street with a thumping majority, few could have imagined how the last few days would play out.

Read more:What will Starmer learn from chaos?PM: Concessions are 'common sense'Analysis: Starmer is in a hot mess

More than 120 MPs, nearly a third of the parliamentary Labour party and more than the total number of Tory MPs, publicly prepared to rebel on a flagship policy.

How did it come to this? How did the prime minister, and the people around him, not see a rebellion coming when there had been signs MPs weren't happy for weeks?

Those are the questions being asked by senior Labour figures behind the scenes.

Sir Keir's spokesperson says the prime minister consistently engages with colleagues, and parliamentary engagement takes many forms.

But a lack of engagement with backbenchers has led to the prime minister's most damaging U-turn yet, and this week will haunt the prime minister beyond Tuesday's crunch vote.