"He told me I should sue the European Union", she said, adding: "Not go into negotiations - sue them".
In a press conference Friday at Chequers, May's country retreat, Trump said he had offered advice to May about Brexit, leaving journalists wondering what that advice had been.
She added: "But the legacy of Brexit can not be a hard border between Northern Ireland and Ireland that unpicks the historic Belfast Agreement".
"Actually no, no we're going onto negotiations with them".
The Mail on Sunday said Conservative Brexiteers are sharpening their knives for the prime minister amid swirling rumours that they will oust her as leader unless she abandons her "soft Brexit" plans.
In an explosive interview with The Sun newspaperpublished Thursday - just as May was hosting Trump at a lavish black-tie dinner - Trump said the British leader's approach likely "killed" chances of a free-trade deal with the United States.
And when pressed in a newspaper interview afterwards, Trump still declined to reveal what his suggestion was. Steve Baker, the Brexit minister who quit along with Davis, accused her in The Sunday Telegraph of presiding over a "cloak and dagger" plot to undermine Brexit. May try in her discussions with Brussels over the terms of Britain's planned exit from the EU. I wouldn't say advice.
At a joint press conference on Friday, Trump softened his criticism of May's leadership - though he didn't back off from his central warning on trade.
Asked to rate USA -U.K. relations, Trump gave them the "highest level of special". He added it was up to Mrs May how to handle Brexit, as long as the USA "can trade and we don't have any restrictions" on commerce with the United Kingdom.
May said that Trump, who had been in Britain for a visit, had noted that since she's already entered negotiations, she should not walk away. "Sue the EU", May told BBCtelevision.
A White Paper published on Thursday fleshed out details of her plan, which advocates close links with the European Union on trade in goods, but not services.
There was also a rally on Saturday in London by Trump supporters and right-wing activists who wanted to show their support of Trump.
Mrs May faces accusations the British government's Brexit White Paper was "surreptitiously" drafted in order to replace a version produced by David Davis's Brexit department.
The British prime minister made the revelation as she tries to unite her Conservative Party behind her blueprint for a United Kingdom withdrawal from the European Union, warning detractors that failing to "keep our eyes on the prize" could result in "no Brexit at all".
"We need to keep our eyes on the prize".
Mrs May also turned her focus to a crucial vote in the House of Commons on Monday, in which both europhile and eurosceptic Conservative MPs could rebel against the government's Trade Bill, a key piece of Brexit legislation.