Theresa May has revealed that Donald Trump advised her to “sue the European Union” rather than negotiate with the 27-country bloc, in a private conversation that the US president referred to during his visit to the UK on Friday.

The British prime minister was asked on the BBC’s The Andrew Marr Show what the “brutal” Brexit negotiating advice was that Mr Trump had talked about in their joint press conference outside the prime minister’s Chequers country retreat.

Revealing it for the first time, Ms May said: “He told me I should sue the EU.” After being prompted by a surprised Marr, Ms May repeated: “Sue the EU, not go into negotiations with them, sue them.”

The prime minister smiled, and indicated she had disregarded the advice, saying “actually we’re going into negotiations with them”, in remarks that will be interpreted as a put-down of the president. Mr Trump leaves the UK on Saturday afternoon to fly to Helsinki to meet Russian president, Vladimir Putin.

On Friday, Mr Trump had said he gave Ms May “a suggestion, I wouldn’t say advice” about how to handle the Brexit talks, without revealing what it was. “I think she found it maybe too brutal, and that’s OK. I gave her a suggestion, not advice. I could fully understand why she thought it was a little bit tough.”

Marr’s interview with Ms May comes at a politically perilous moment for the British prime minister, who is facing calls for a confidence vote over her leadership of the Conservative party, and the prospect of her former foreign secretary Boris Johnson spelling out the reasons for his resignation in a newspaper column on Monday.

Britain’s prime minister Theresa May speaks to Andrew Marr (left) on the BBC’s Andrew Marr Show on Sunday

Reports at the weekend suggested that around 40 of the 48 MPs needed had lodged no-confidence letters with the chairman of the party’s backbench 1922 Committee.

Ms May indicated that she would seek to fight off any challenge, saying: “I want to focus people’s minds on how you ensure you achieve that prize, the benefits of leaving the European Union.” She added: “I have always said I’m in this for the long term.”

Embarrassed

The British prime minister was initially embarrassed by Mr Trump when he gave an interview to the Sun, published on the morning of the press conference, in which he appeared to say that Ms May’s Brexit plan would prevent a trade deal with the US and said Mr Johnson would make a good prime minister.

Mr Trump subsequently apologised to May in private and partially backtracked on the remarks on Friday, saying that the UK should pursue its own Brexit policy and trade negotiations but: “Just make sure you can trade with us.”

Marr also asked the prime minister if she believed Mr Trump had “a medical problem” with stairs because he frequently takes her hand when the pair find themselves facing a step or two. Ms May acknowledged that “whenever he takes me down a slope, or stairs ... he takes my hand to help in going up the steps”.

Donald Trump leads Theresa May by the hand as they climb the steps to the entrance of Blenheim Palace, where they attended a dinner this week. Photograph: Will Oliver/Reuters

The BBC interviewer then asked her if that made her look submissive. “Oh, Andrew come on,” Ms May replied. “Are you telling you have never said to somebody: let me help you?”

She also said that she believed she could trust Mr Trump at his word, and in an echo of language used by the president, added that the UK continued to enjoy “the highest degree of special relationship with the United States”.

Earlier, Ms May warned against a post-Brexit hard border in Ireland that would “unpick” the Belfast Agreement.

“I know there are some who have concerns about the ‘common rule book’ for goods and the customs arrangements which we have proposed will underpin the new UK-EU free trade area. I understand those concerns,” Ms May wrote in an article for the Mail on Sunday.

“But the legacy of Brexit cannot be a hard border between Northern Ireland and Ireland that unpicks the historic Belfast Agreement.

“It cannot be the breaking up of our precious United Kingdom with a border down the Irish Sea. And it cannot be the destruction of integrated supply chains and just-in-time processes on which jobs and livelihoods depend.” - Guardian, PA