Rayner is powerhouse in Labour Party - and her departure creates difficulties | Beth Rigby

It was clear from the prime minister's very personal and handwritten letter to his fallen deputy Angela Rayner that this was a resignation he did not want and deeply regretted.

And so he should: Ms Rayner's departure from his government is a huge loss. She is a powerhouse within the Labour Party and a figurehead of the left who brought Sir Keir's project some legitimacy from quarters of the Labour movement minded to give him short shrift.

Describing her as a "true friend" and the embodiment of the sort of social mobility he would want to leave as his legacy in office, it was clear that the prime minister both mourned her departure and wanted to keep his formidable and politically astute former deputy onside. Because her departure does create difficulties.

She is badly-wounded now but could be a beacon on the backbenches should she seek to rebuild her political career and climb the ladder once more. Ms Rayner is a politician deeply admired in pockets of the party and a product of the trade union movement.

Politics latest: More changes to PM's top team expected after cabinet reshuffle There is no sense that she would be disloyal to Sir Keir but she is a figurehead whether she wants it or not. The manner of her departure - forced out because she underpaid her tax and in doing so failed to uphold the highest standards in public office - is also difficult.

Sir Keir's rivals will be armed with plenty of ammunition this Budget should the chancellor choose to raise property taxes even as a leading Labour figure underpaid theirs. There is also the matter of the deputy Labour leadership race: this is an elected post that Ms Rayner quit on Friday and already there is a battle emerging between the party's left and centre left over who should succeed her.

As the saying goes, "divided parties don't win elections.