• Video report by ITV News Correspondent Juliet Bremner

Donald Trump was unaware Britain was a nuclear power and came closer to withdrawing the US from Nato than is widely believed, according to leaks from a new book by his former national security advisor.

John Bolton, a staunch conservative and former UN ambassador who served for 17 months in the White House, said the president is "stunningly uninformed" in his new memoir, The Room Where It Happened.

Mr Bolton was fired by Mr Trump due to "strong disagreements".

The Times newspaper reported the book has been leaked to the media despite efforts by the Trump administration to block its publication which is planned for June 23.

A spokesperson for Mr Trump said the book should not be published as it contains "classified" information and to do so would be "unacceptable".

Donald Trump at a visit to Downing Street with then PM Theresa May. Credit: AP/Twitter/@rianru

One excerpt told of a 2018 meeting with then Prime Minister Theresa May in which a UK official referred to Britain as a nuclear power.

Mr Trump is said to have replied: "Oh, are you a nuclear power?"

Mr Bolton makes it clear the line was not intended as a joke.

The former ambassador also wrote that at a Nato summit in 2018 Mr Trump had decided to tell allies the US would pull out of the group if other countries did not increase their defence spending.

President Trump at a working lunch with NATO members at the end of last year. Credit: AP

The book claims the president told Mr Bolton in a message: "We will walk out, and not defend those who have not (paid)."

Mr Trump, however, ultimately did not make such a dramatic threat, with the US staying in the alliance.

Mr Bolton said Mr Trump also asked Chinese President Xi Jinping to help his re-election chances - and had on occasion expressed a willingness to stop criminal investigations involving companies in China and Turkey in order to "give personal favours to dictators he liked".

Speaking on Wednesday evening, US Trade Representative Robert Lighthizer said the request for help with re-election "never happened".

It would not be the first time President Trump has been accused of using bargaining tools for personal advantage - his impeachment trial earlier this year focused on a similar charge.

NATO leaders last met in person in December 2019 in the UK. Credit: PA

In the memoir, Mr Bolton said the president's impeachment charge, for which he was acquitted, had indeed been true.

He claims Mr Trump had tried to leverage US aid to pressure Ukraine to investigate the son of his rival Joe Biden.

Mr Bolton controversially refused to testify at the impeachment trial, however, instead saving his version of events for the memoir - a book deal reportedly worth £1.5 million.