Angela Rayner has been accused of a bid to block Sir Sadiq Khan from a return to parliament through new legislation.
A bill tabled by the deputy prime minister would prevent the mayor of London from becoming an MP while still in office.
The bill would thwart any chance of Sir Sadiq returning to parliament before the next general election, ending the prospect of a bid for the leadership if Sir Keir Starmer steps down.
Ms Rayner’s department said the change, which would apply to other mayors including Andy Burnham, would ensure they are focussed on serving their cities.
But a Tory strategist told The Sunday Telegraph: “Fresh out of trying to do in Wes Streeting, yet again Angela Rayner is on manoeuvres, knifing leadership rivals. She should focus on delivering for Britain rather than on her petty party political squabbles. Yet again it’s party first, country last.”
Boris Johnson was an MP while serving as mayor of London, having used the mayoralty to raise his profile before his eventual leadership bid.
The English Devolution Bill put forward by Ms Rayner would prevent Sir Sadiq from following a similar path to Mr Johnson, with speculation he is interested in the leadership.
Sir Sadiq has denied he wants to become Labour leader, stating he has “no interest at all” in the position and that he has “no ambitions to go back to the Commons”. He is currently serving his third term as London mayor after last May’s election and is not expected to run for a fourth.
The Independent has contacted Mr Khan for comment.
After a disastrous first year since the general election, which saw his poll ratings plummet, Sir Keir has faced growing speculation his MPs will seek to oust him as leader.
A growing number of MPs on the “soft left” of the Labour Party have been vocally critical of the PM, with some urging Ms Rayner to launch a bid to replace him.
Ms Rayner has previously said she never wants to be leader of the Labour Party or prime minister, insisting she is happy with her current job.
In a recent Sky News interview, she said: “Absolutely not. I don’t want to be leader of the Labour Party.”
She added: “I’m very happy and honoured to be deputy prime minister of the country, and I’ve got a lot in my in-tray to prove that I can do the job that I’m doing and deliver on those milestones for people in the country. That’s what I’m interested in.”
A spokesman for Ms Rayner’s Ministry of Housing, Communities and Local Government said: “It is already the case that mayors who are police and crime commissioners cannot also serve as MPs, and these changes will make this consistent for all mayors so that they can focus on what is a significant role at the forefront of delivering change.”
