One minister told The i Paper that Mr McSweeney has “never really gone away” from Sir Keir’s inner circle since stepping down from his role in No10.

“He has been on the end of the phone 24/7,” the source said.

The insider added that Mr McSweeney had been “wargaming” a leadership contest and believes Sir Keir could still prevail if the field becomes crowded.

“He has been working out how the votes could go. It’s mad,” the source added.

Separately, three Labour sources told The i Paper that Mr McSweeney and the Prime Minister are now in regular contact, while a fourth Whitehall insider claimed the former chief of staff had recently been seen inside No10.

Those close to Sir Keir sought to play down suggestions of a formal return, dismissing reports that Mr McSweeney had been back inside Downing Street as “rubbish”.

One minister revealed that Mr McSweeney has ‘never really gone away’ from Sir Keir’s inner circle

However, they declined to say how frequently the pair speak or when they last spoke.

Mr McSweeney’s apparent return to influence is likely to anger some of the Prime Minister’s internal critics.

Critics have also pointed to what they describe as a “boys’ club” culture within No10 during his time as Chief of Staff.

Many blame him for Labour’s current difficulties, including his role in recommending Lord Mandelson for the Washington ambassadorship.

The insider said that Mr McSweeney had been ‘wargaming’ a leadership contest and believes Sir Keir could still prevail

 | GETTY

Supporters of Mr McSweeney, however, argue he deserves significant credit for Labour’s landslide victory at the 2024 general election and remains one of the party’s most effective strategists.

The pair reportedly met for a drink shortly after Mr McSweeney’s departure in February, according to Politico.

He is also said to have advised Sir Keir during the period in which Andy Burnham announced his intention to seek a return to Parliament.

The Prime Minister previously acknowledged that he spoke to Mr McSweeney “every few weeks”, although sources suggest contact has increased significantly in recent months.

The Greater Manchester Mayor confirmed last week that he would enter any future leadership contest should he win the seat

 | GETTY

Upon his resignation, Mr McSweeney accepted “full responsibility” for recommending Lord Mandelson’s appointment.

He said the decision had damaged confidence in Labour, the country and politics more broadly, making his departure the “honourable course”.

His exit was regarded as a major blow to Sir Keir, who had become heavily reliant on his political judgement.

Several Labour figures describe Mr McSweeney as the Prime Minister’s “political brain”, making his continued involvement unsurprising to many within the party.

One MP offered a blunt assessment: “If McSweeney is back it just goes to show the PM has learned nothing.”

Pressure on the Prime Minister has intensified following the resignations of Defence Secretary John Healey and Armed Forces Minister Al Carns over military spending.

The timing is particularly awkward, with the G7 summit in France beginning on Monday.

Attention is now turning to Thursday’s Makerfield by-election, which could see Mr Burnham return to Westminster.

The Greater Manchester Mayor confirmed last week that he would enter any future leadership contest should he win the seat.

Speaking to the BBC on Friday, Sir Keir said he did not want to “plunge” Britain into the “chaos” of a Labour leadership contest.

However, he added: “If it does happen, I will fight.”