Sir Keir Starmer will ‘seriously cripple’ the UK’s ‘most important relationship’ if he becomes prime minister, an expert has claimed.

Professor Azeem Ibrahim, a former adviser to the Biden campaign, said Sir Keir’s previous vocal opposition to Donald Trump could “seriously cripple” the UK’s relationship with the US.

Professor Ibrahim, who published a detailed report on Keir Starmer’s foreign policy earlier this month, told Express.co.uk: ‘When it comes to post-Brexit Britain, the world is just going to be divided into big camps – and those camps are going to be the United States, China and maybe India.

“Europe is going to be much less important.

“After Brexit, Britain, our interests are much more aligned with the United States.

“And Starmer has been very critical of Donald Trump in the past and rightly so, but he hasn’t indicated that he will be able to work with a Republican president.

“The Republican Party in the United States has changed quite dramatically, and Starmer’s criticisms of Trump and Republican positions are well-spoken.

“A Republican president is very likely in the next election and if you look at the frontrunners, they contain a lot of elements of Trumpism.

“So I think Starmer really needs to signal that he is able and willing to work with a Republican party if he takes control, otherwise our most important post-Brexit relationship could be seriously handicapped.”