Labour MP Catherine West has said if a cabinet minister does not challenge Sir Keir Starmer as party leader by Monday, she will attempt to trigger a leadership contest herself.
In an exclusive interview with the BBC, she said her preferred option would be for the cabinet to "reorganise themselves" and put forward their "best communicator" to replace Sir Keir, avoiding a leadership election.
But speaking after devastating election results for Labour, West said she was putting the cabinet "on notice" and if she did not hear from a leadership hopeful by Monday she would ask Labour MPs to back her to trigger a contest.
For this to happen, 20% of Labour MPs – 81 people – would have to support her.
West, a former junior Foreign Office minister, said she currently had 10 MPs prepared to back her in a leadership bid but she was "confident" enough people would come forward to trigger a contest.
"My preferred option is for the cabinet to do a reshuffle within itself, where there's plenty of talent, and for Keir to be given a different role, which he might enjoy, perhaps an international role," the MP for Hornsey and Friern Barnet told Radio 4's PM programme.
"Then for others to come to the fore who can communicate the message, who are very able, so we can have minimum fuss."
However, Cabinet Office Minister Nick Thomas-Symonds said he would "caution colleagues" against her proposal.
"We've seen over the past 10 years now, what happens when a party in government just starts chopping and changing leaders," he told the programme.
"It just generates instability and it militates against a focus on delivery."
While West is not trying to become Labour leader herself, she could tempt others to launch a leadership bid.
But if her plan falls apart and prompts Labour MPs to row in behind Sir Keir, his position could be strengthened.
Sir Keir has insisted he will not "walk away and plunge the country into chaos", after Labour suffered heavy losses in elections for English councils, as well as the Welsh and Scottish parliaments.
The prime minister is planning an attempt to reset his premiership this week by delivering a major speech and unveiling a new programme of legislation.
In interviews with the Observer and Mirror newspapers he said he would be "full throated" about his desire to get closer to the EU.
He will seek to draw bold dividing lines with Reform UK by saying he won't hold back from seeking closer ties to Brussels and that he expects to launch a scheme to allow young EU citizens to work temporarily in the UK - and vice versa - next year.
Cabinet ministers rallied round the PM on Friday evening, voicing their support on social media.
However, around 30 Labour MPs have publicly called for a change of leader or a timetable for Sir Keir to go, since the results started coming in on Friday.
Labour has lost more than 1,460 seats in English council elections, with Reform UK making huge gains in former Labour heartlands in northern England and the Midlands.
Meanwhile, the Greens won control of Waltham Forest, Lewisham and Hackney in London, where Labour had previously been dominant, as well as winning the party's first-ever elected mayors in Hackney and Lewisham.
In Wales, Labour suffered a historic defeat in the Senedd, finishing third behind Plaid Cymru and Reform UK.
There were also heavy losses for the party in the Scottish Parliament election, where the SNP won a fifth successive term and Labour finished a distant second tied with Reform.
West did not name who she thought would be best replacement for Sir Keir.
"I don't have a candidate. That's part of the problem," she said.
"But I think there are several people who would like to do it, who have been planning for months, but I'm very surprised that none of them has popped up today to say 'I will do it'."
West said Sir Keir had done "excellent" work rebuilding the Labour Party and on foreign policy.
But she said he had "less of a grip" on the domestic agenda and the party needed to "take the fight to Reform".
West's intervention appeared to take many Labour MPs by surprise.
One minister - who is critical of the prime minister - told the BBC: "She's mad."
A prominent backbench critic of Sir Keir said it was a "totally irresponsible solo mission".
Immigration Minister Mike Tapp was also critical of West's proposal, saying: "When those within your own walls begin dismantling the gate, the enemy no longer needs a battering ram.
"Reform are loving it. Awful from Catherine West and she should know better."
One Labour MP, who is not a prominent critic of Sir Keir, told the BBC that while they had been blindsinded by West's interview, they would be giving her their support on Monday.
"I am reasonably confident she will be able to get to 81 [backers]," they said.
"The frustration on the backbenches runs far wider than the voices we've heard from publicly. There are far more moderate centrist Labour MPs who also think his time is up."
However, Downing Street sources do not think she will reach that threshold.
Some in government believe West's call for the cabinet to replace Sir Keir with its "best communicator" means she is tacitly supporting Wes Streeting, though this is denied by those close to the health secretary.
On Friday, Streeting said Sir Keir had his support but "we have to take responsibility in government for our mistakes".
Asked by the BBC if he was ruling out a leadership bid, Streeting said Sir Keir had "delivered a majority that people thought was absolutely impossible after the crushing defeat of 2019".
Other names touted as potential leadership candidates outside the cabinet include former Deputy Prime Minister Angela Rayner, who resigned in September over failing to pay enough tax on a flat purchase.
However, she is believed to still be waiting for the conclusion of an HMRC investigation into her tax affairs and may prefer to wait for its findings before launching a leadership bid.
Greater Manchester mayor Andy Burnham has the support of many MPs but any leadership contender must be a member of Parliament.
Earlier this year he was blocked from standing as a candidate in the Gorton and Denton by-election and he would need to find another seat to become an MP, as well as get the approval of Labour's ruling body, the National Executive Committee.
Burnham supporters had hoped to prop the PM up temporarily but encourage him to announce a timetable for his departure, which would allow the mayor to find a parliamentary seat.
They have tried to persuade West - who is also on the centre-left of the party - to get on board and abandon plans to provoke a challenge now.
However, as things stand, she will proceed on the grounds it would take too long for Burnham to return to Westminster.
In an attempt to shore up his position, Sir Keir on Saturday appointed former Labour Prime Minister Gordon Brown and former deputy leader Harriet Harman as advisers.
The veteran politicians will advise the PM on global finance and tackling violence against women.
However, a number of Labour MPs expressed bafflement at the appointments.
One normally loyal minister told the BBC: "It's a joke. There is no question to which bringing these two back is the answer."
Paula Barker, the Labour MP for Liverpool Wavertree, told the BBC: "I have enormous respect for both Gordon and Harriet, but I would have had even more respect for them if they had declined the offer of, quite frankly, non-jobs and told the prime minister that it's time for a change and he should set out his timetable."
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