Mike Pence would have supported attempts to remove Donald Trump from office under the 25th amendment, according to a senior White House official.

The anonymous author of a new book titled A Warning raises the possibility of the president being declared unfit due to mental incapacity.

This step – which has never been taken before in US history – would require the approval of the vice-president and a majority of the cabinet.

The official claims that senior figures in the administration believed that Mr Pence would go along with a cabinet plan to invoke section four of the amendment.

However the author does not claim to have received direct confirmation – and the vice president himself rejected the idea when questioned by reporters on Thursday.

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“I never heard anything in my time as vice president about the 25th Amendment,” Mr Pence replied. “And why would I?”

The vice president also described the book as “appalling”.

Discussions about invoking the 25th amendment are said to have taken place soon after Mr Trump sacked FBI director James Comey, who was leading an investigation into links between Mr Trump’s campaign team and Russia.

Last year the anonymous official wrote in an opinion piece for The New York Times that there were “whispers” among members of the cabinet about invoking the 25th amendment.

Mr Trump reacted furiously by demanding the newspaper identify the author, describing the anonymous official as “gutless”.

“Does the so-called ‘Senior Administration Official’ really exist, or is it just the Failing New York Times with another phony source?”, he tweeted.

White House press secretary Stephanie Grisham said the book was “a work of fiction”.

“The coward who wrote this book didn’t put their name on it because it is nothing but lies,” she told The Washington Post. “Real authors reach out to their subjects to get things fact checked – but this person is in hiding, making that very basic part of being a real writer impossible.”

Meanwhile the impeachment inquiry is seeking to investigate how much Mr Pence knew about efforts by Mr Trump to pressure Ukraine to investigate former Vice President Joe Biden.