Sir Keir Starmer is to promise bolder action to tackle "big challenges" facing the country, as he battles to persuade his MPs not to ditch him as Labour leader.
The prime minister will seek to reset his premiership in a speech on Monday, after heavy election losses left him facing renewed calls to quit.
Growing numbers of Labour MPs went public with calls for him to go over the weekend, with potential challenger Angela Rayner saying the party needed to do more to appeal to voters whose living standards had been squeezed.
It comes as backbencher Catherine West said she could try to trigger a leadership contest in the wake of his speech.
The north London MP has threatened to begin canvassing support if Sir Keir's address leaves her "dissatisfied" with his plan to turn the party's fortunes around.
Although she has made clear she is not seeking to replace the prime minister herself, her move could provide a route for other contenders to come forward, including former Deputy Prime Minister Rayner and Health Secretary Wes Streeting.
Greater Manchester mayor Andy Burnham is also seen as a potential leadership rival, although a speedy contest would hamper his chances because he would first need to become an MP to stand as a candidate.
Monday's address could prove to be the most important speech of Sir Keir's premiership so far, after the disastrous results at this week's elections left his leadership position more vulnerable than ever.
At the heart of the speech will be an ambition to put Britain "at the heart of Europe" by seeking closer ties with the EU, a move he said would "define" his government and strengthen the UK's economy, trade and defence.
The prime minister will also say that the government's legislative plans for the coming year, to be unveiled at the King's Speech on Wednesday, will show "hope, urgency and exactly whose side we are on".
"We will face up to the big challenges and we will make the big arguments," he is expected to say, according to extracts released by Downing Street.
"To meet the challenges that our country faces incremental change won't cut it," he will add.
"On growth, defence, Europe, energy – we need a bigger response than we anticipated in 2024 because these are not ordinary times."
In an intervention on Sunday, Rayner stopped short of launching a leadership bid herself but warned that Labour faced its "last chance" to repair its relationship with voters facing "squeezed living standards".
In a lengthy statement, she called for "immediate action to cut costs for households," action to help struggling high streets, a rising minimum wage and more public and community ownership "across the board".
She also backed Andy Burnham to return to Westminster, adding the party had been wrong to block him from standing in a by-election in February and Labour needed to bring "our best players into Parliament".
Labour lost almost 1,500 councillors in local elections across England amid a surge for Nigel Farage's Reform UK, with the Greens also eating into the party's support in London and other urban areas.
It was also kicked out of power in Wales, where it has enjoyed political dominance for a century, and returned just 17 of the 129 seats in the Scottish Parliament, its worst ever result at a Holyrood election.
The polls, the biggest set of elections since Labour's landslide election victory in 2024, had been billed as a key test for Sir Keir amid dire poll ratings for Labour.
More than 30 Labour MPs have publicly called for the prime minister to resign or to set out a timetable for his departure in the wake of the results.
However the party's rulebook presents a hurdle for potential leadership hopefuls, with contenders required to win the endorsement of 81 Labour MPs, a fifth of the current total, in order to trigger a contest.
Burnham's supporters are hoping a leadership contest can be delayed until he is able to return to Parliament. As things stand, no MP has said they would be prepared to vacate their seat to pave the way for him to return.
Rayner resigned as Sir Keir's deputy prime minister last September after it emerged she had failed to pay the correct amount of stamp duty on a £800,000 flat in Hove.
It has been widely reported that she is still waiting for the result of an HMRC investigation into the purchase, which could complicate any immediate leadership campaign.
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