Wes Streeting has now gathered sufficient support from Labour MPs to mount a leadership bid against Sir Keir Starmer, with peers pushing for him to act within days.
The Health Secretary has secured backing from more than 81 party colleagues, crossing the threshold needed under party rules to enact a contest for leadership.
Labour's constitution requires a minimum of 20 per cent of MPs to publicly endorse a challenger, with 81 signatures deemed enough given the current parliamentary party has 403 members.
Supporters of Mr Streeting are urging him to make his move as early as next Friday, immediately following the May 7 local elections.
By enacting a challenge at this time, the current Health Secretary would be able to leverage what is expected to be a devastating result for Labour in councils across Britain.
Awkwardly, the Prime Minister became aware of the brewing challenge through an mishap involving one of his own staff.
A Downing Street employee received a text message by mistake, containing details of Mr Streeting's campaign strategy including references to "five pillars" underpinning his pitch and a "PFG" (plan for government).
Should Mr Streeting proceed, he faces a choice between formally declaring his candidacy or resigning from Cabinet in hopes of triggering a wave of departures to pressure the Prime Minister into standing down of his own accord.
Wes Streeting has secured backing from more than 81 party colleagues
| PAAn early strike would catch potential leadership rivals off guard, particularly Manchester Mayor Andy Burnham, who cannot stand as he lacks a Westminster seat.
Senior Labour figures are bracing for electoral devastation when results come in next week.
One well-placed party source told The Telegraph: "I don't think anyone really understands the scale of what is going to happen next week.
"Labour is going to lose in places it has never lost, including in parts of London. It will be destroyed in the Midlands and the North, and once the northern barons turn against Starmer, it's over. It will be carnage."
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Approximately 5,000 council seats are up for grabs across England, alongside elections to the Scottish Parliament and Welsh Senedd.
Some within the party believe Sir Keir should announce a departure timeline, potentially stepping aside after September's Labour conference to allow an orderly transition.
Yet many MPs refuse to grant him such breathing room, with signs his grip on power could collapse rapidly once votes are counted.
Labour is expected to face a landslide defeat in the May 7 local elections
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Despite having the numbers, Mr Streeting remains cautious about striking first.
Historical precedent suggests those who bring down leaders seldom inherit the title themselves, weighing up alternative options such as allowing rivals to show their hand or waiting to see whether Sir Keir departs voluntarily.
Angela Rayner has yet to decide on her own intentions, reportedly engaged in discussions with Mr Burnham and Energy Secretary Ed Miliband about forming a soft-left alliance to seize control.
Meanwhile, Downing Street advisers are divided over strategy, with acting chief-of-staff Vidhya Alakeson advocating an immediate Cabinet reshuffle, while political director Amy Richards warns such a move would only create additional problems.
Amid the ruckus, a spokesman for Mr Streeting maintained he "supports the Prime Minister" and remains "completely focused on his job".
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