Keir Starmer is struggling to convince world leaders he is going to survive – despite telling his ministers he will 'fight to win' against any leadership challenge.

Diplomatic sources told the Daily Mail that the European Commission is resisting setting a date to discuss Sir Keir's 'Brexit reset' because of doubts about whether he will remain in post to implement any agreement.

One said: 'There is some goodwill towards him because he has shown he is sincere about thawing relations after Brexit and he has proved a reliable partner on Ukraine.

'But at the same time, everyone can see he has big domestic problems. People are asking, what is the point of doing a deal with someone who could be gone in a few months? Why not wait for the next guy?'

Sir Keir made the promise of closer relations with the EU a central plank of his 'fightback' speech after the local elections.

Aides had hoped to schedule a summit around the date of the tenth anniversary of the Brexit referendum on June 23 and are now trying for July.

But officials have so far failed to agree a date amid growing speculation it could be pushed back to the autumn.

Whitehall sources acknowledged that fixing a date had proved difficult but suggested that Brussels is stalling in order to put pressure on the UK in the negotiations on issues including a youth mobility scheme.

Doubts about Sir Keir's future have also led to what one minister described as a 'go slow' by civil servants who are waiting to see if a change of leadership leads to a change of direction.

Sir Keir is fighting desperately to save his job in the wake of last month's catastrophic local elections.

On Monday he called in several dozen junior ministers thought to be loyal to tell them he would fight any leadership challenge by Andy Burnham or Wes Streeting.

'If there is a contest, then I'm going to fight it,' he told them. 'I'm not going to walk away. I am going to fight to win. I'll be in it to win it.'

Allies of the PM have even briefed reporters that Sir Keir's wife Victoria has told him to 'carry on' despite 100 Labour MPs urging him to quit.

Mr Burnham's supporters believe he will launch a leadership challenge within weeks if he triumphs in next week's Makerfield by-election.

He is said to be determined to replace the PM in time for Labour's annual conference in Liverpool in September.

Labour MPs believe he will try to persuade Sir Keir to step aside, potentially allowing him a 'coronation'. Some think he could try to dislodge the PM by offering him the chance to stay on as foreign secretary.

But former health secretary Wes Streeting has vowed that he will trigger a contest.

And some supporters of Sir Keir are urging him to stand and fight for his job. Mr Burnham's public declaration that he wants the PM's job has infuriated allies of Sir Keir.

They are trying to persuade wavering MPs to hold off from backing a challenge – and point to Mr Burnham's difficulties on the campaign trail.

The Greater Manchester mayor has made a string of U-turns on issues ranging from immigration to trans rights as he tries to hold off a challenge from Reform.

Last week he raised eyebrows by refusing to answer questions about the details of Labour's fiscal rules.

He insisted that he understood the rules, which govern spending and borrowing, but told the BBC: 'I'm not going to go through a discussion like an exam on the fiscal rules.'