UK Prime Minister Keir Starmer pledged on Tuesday to never walk away from his job to change Britain, brushing off a challenge to his authority by the Labour leader in Scotland and other figures in the party who have called on him to quit.

Under pressure over his appointment of Peter Mandelson as ambassador to Washington because of the Labour veteran's close ties to the late US sex offender Jeffrey Epstein, Starmer came out swinging, urging his party to fight the real enemy – the populist Reform UK party – rather ⁠than among themselves.

He used a visit to a community centre in southern England to try to show that his political career was anything but dead, a day after the biggest challenge to his authority when Scottish leader Anas Sarwar called on him to quit and he lost a second aide in as many days.

"I will never walk away from the mandate I was given to change this country," Starmer told an audience. "I will never walk away from the people that I'm charged with fighting for, and I will never walk away from the country that I love."

Starmer said the real fight was with "the politics of Reform, the politics of divide, divide, divide, grievance, grievance, grievance that will tear our country apart."

He won a reprieve late on Monday after winning the backing of his top team of ministers, the support from potential leadership rivals such as former Deputy Prime Minister Angela Rayner and from many of his Labour lawmakers in parliament.

On ⁠Tuesday, another possible leadership contender, Greater Manchester Mayor Andy Burnham, also said he supported Starmer.

"Yes, he has my support, the government has my support," he said after a speech to the Resolution Foundation think-tank.

"I think what we all need to do now is to drive the pace of change more quickly and that means greater unity across the whole family of the Labour movement."

Revelations about the depth of Mandelson's relationship with Epstein have spurred the greatest threat yet to Starmer, who has repeatedly said the former ambassador and Labour veteran had lied over his ties to the late sex offender.

Last month, the US Justice Department included emails suggesting Mandelson had leaked discussions on possible British asset sales and tax changes to Epstein during the financial crash.

Mandelson has not commented publicly on allegations he leaked documents, and has not responded to messages seeking comment.

He ⁠is now under police investigation for alleged misconduct in office. (Reuters)