Keir Starmer admitted Labour leadership rumours are 'rife' today as he was battered by MPs over briefings against the Cabinet and Budget leaks.
The PM squirmed as he was pressed on the behaviour of his own No10 aides during an intense session at the powerful Commons Liaison Committee.
Sir Keir was repeatedly challenged over how he would deal with allies who were operating unacceptably.
At one point he was asked if Labour leadership speculation was a 'purely party political' for the purposes of conduct rules.
'No, I'm not sure I can,' he said ruefully. 'It seems to be pretty rife.'
The exchanges came after an apparent 'hit job' designed to curb manoeuvring by Wes Streeting backfired last month, with the furious Health Secretary publicly calling for those responsible to be sacked.
Keir Starmer squirmed as he was pressed on the behaviour of his own No10 aides during an intense session at the powerful Commons Liaison Committee
Chancellor Rachel Reeves has also been accused of allowing pre-Budget forecasts to be selectively leaked to help soften up Britons for massive tax hikes - something she denies.
Sir Keir insisted today that briefing against Cabinet ministers was 'completely unacceptable'.
The PM said he had 'no reason' to think No10 had been involved in the briefing, but stressed he would act if necessary.
'Obviously, I'll wait for the inquiry, but I mean the inquiry will go wherever the evidence takes it,' he said.
'There's nothing to inhibit where they will go, and I'll take appropriate action if there's a finding.'
He added: 'I'll get to the bottom of these leaks. They are in any organisations, they're intolerable.
'I took the same action when I was head of the Crown Prosecution Service. There is a leak inquiry. It can go wherever the evidence will take it, and if it comes to a conclusion, I'll act on it.'
Sir Keir said special advisers had assured him that briefing against Mr Streeting had not come from Downing Street.
'I didn't just take everything at face value. I looked at other bits of evidence to assure myself about what I was then saying publicly to the media,' he said.
He said he had been 'clear and consistent' with his staff that briefing against Cabinet ministers is 'completely unacceptable'.
Sir Keir's woes have been fuelled by apocalyptic polls and the big-tax Budget landing poorly with the public.
Andy Burnham, Wes Streeting and Angela Rayner are among the senior figures regarded as jockeying for a post-Starmer era.
However, a major problem for rebels is that 80 MPs need to unite behind a single challenger in order to spark a leadership contest.
Andy Burnham has been at the centre of intense leadership speculation, with allies saying he is poised to make a comeback to the Commons
Sir Keir's fraying grip on party discipline was underlined today as a serving minister launched a devastating public attack on his management skills and judgment.
Ian Murray complained he still does not know why the PM demoted him out of the Cabinet in a chaotic reshuffle in September.
The Edinburgh MP said he has still received 'no explanation' from Sir Keir why he was axed as Scottish Secretary.
He also revealed he considered walking away altogether rather than taking the more junior job he was offered - admitting his wife was 'f***ing furious' with the premier.
