One of Labour’s largest donors has pledged to stand against Jeremy Corbyn in the General Election if he does not stand down.
Michael Foster, who has given more than £400,000 to the party since 2010, said Labour faces “annihilation” with Mr Corbyn at the helm.
The challenger levelled a startling accusation that supporting the current leader “means supporting a political strategy that kills people” – linking Tory dominance to longer heart operation wait times.
He said he would be “very happy” to stand against Mr Corbyn, who he branded as “blinkered and ineffective”, in his Islington North constituency ahead of June 8.
"There are a lot of things a political party looks for in a leader. Jeremy Corbyn possesses none of them. Because of him, Labour faces annihilation at the polls,” he said in the Sunday Times.
The agent, who was suspended by the party after launching a legal bid to oust Mr Corbyn, said the results of the looming local elections should decide his fate.
Issuing an ultimatum, he said: "If the results are as bad as predicted, then Jeremy should stand down voluntarily and let someone else lead the fight in June.
"If he does not - and I don't believe he ever would - I would be very happy to fight him and his fellow travellers on the extreme left of the party by standing in his constituency of Islington North."
Mr Corbyn has been MP for the north London constituency since 1983 and goes into the upcoming General Election with a majority of more than 20,000.
Experts have predicted Labour will suffer heavy losses across the country in Thursday's council elections.
The Conservative Party and Liberal Democrats are expected to make significant gains in England and Wales, while north of the border the Scottish National Party and Tories will reap the benefits of strong opinion polls.
Mr Foster said the Tories stand to benefit from Mr Corbyn's "incoherence, weakness and lack of leadership", accusing him of underestimating the support for Brexit among Labour's traditional voters.
Even in the event of an electoral "disaster", Mr Corbyn would not stand down, relying instead on the support of the party membership to fend off a vote of no confidence, Mr Foster predicted.
"Jeremy's activist supporters will ensure this blinkered, ineffective leader is immediately elected again," he said.
He added: "Unfortunately, supporting him as leader means supporting a political strategy that kills people.
"By 1997 after 18 years of Tory government, the waiting time for a heart operation was up to 18 months, and by 2003 Labour had reduced the average wait to just four months.
"But now as waiting lists get longer through lack of Tory investment in the NHS, more people die waiting for their operation, at about the same rate that Corbyn and his fellow travellers make Labour more unelectable."
Mr Foster, who has repeatedly criticised Mr Corbyn's tenure as party leader, launched a failed legal bid to block him from the leadership ballot triggered by a shadow cabinet revolt last summer.
The agent was later suspended by the party after comparing Mr Corbyn's allies to Hitler's stormtroopers, alleging in a newspaper article that he had been smeared as a Jewish donor to the party.
Shadow chancellor John McDonnell called for action to be taken against Mr Foster, branding his comments "appalling".
Labour has been approached for comment.