Chris Mason: On welfare, something will have to give

The rebellion is growing bigger and bigger.

Cabinet ministers have been given lists of Labour MPs to ring. Hit the phones and twist the arms, are the instructions from the prime minister to his senior team.

But, so far at least, it isn't working.

Chancellor Rachel Reeves is involved, arguing how important the changes were to ensure the benefits system was affordable and sustainable and how damaging a defeat would be for the prime minister.

Health Secretary Wes Streeting is involved, having face-to-face chats and trying to persuade people.

Business Secretary Jonathan Reynolds is sharing with colleagues his own family's experience – his eldest son Jack is severely autistic and a recipient of the personal independence payment.

Reynolds is telling colleagues it makes him a profound believer in a welfare state that helps those most in need and argues the government's plans do just that.

The attempt to coordinate this persuasion effort is an indicator of how serious Downing Street take this rebellion and how desperate they are to deal with it.

But the mood of many in the parliamentary party appears to be hardening.

Stiffening the resolve of some, Labour's two most heavyweight mayors have now got stuck in.

Mayor of Greater Manchester Andy Burnham told BBC Newsnight: "When the Parliamentary Labour Party delivers its collective wisdom in such numbers it is invariably right.

"And it is right on this. And I would say to the government, listen to the PLP."

Mayor of London Sir Sadiq Khan has set out his opposition too.

Something is going to have to give – and soon, because the scale of opposition and the government's current plans cannot both last.

Sir Keir faces a test of his own judgement and authority he has not seen before since entering Downing Street.