British Prime Minister Theresa May has warned there may be "no Brexit at all" because of continued attempts to undermine her plan to leave the European Union.

Key points: Ms May said the UK will take a tough stance in next round of negotiations with EU

Ms May said the UK will take a tough stance in next round of negotiations with EU Former Brexit secretary David Davis said Ms May's plan would leave the UK open to EU retaliation

Former Brexit secretary David Davis said Ms May's plan would leave the UK open to EU retaliation Donald Trump says the Queen thinks Brexit is "a very complex problem"

"My message to the country this weekend is simple: we need to keep our eyes on the prize," Ms May wrote in the Mail on Sunday newspaper.

"If we don't, we risk ending up with no Brexit at all."

Earlier this week, two senior ministers resigned in protest at Ms May's plans for trade with the EU after Britain leaves the bloc next March.

Her blueprint was then criticised in a newspaper interview by US President Donald Trump, a position he backtracked on during a meeting with Ms May on Friday.

Ms May also wrote in the Mail on Sunday article that Britain would take a tough stance in its next round of negotiations with the EU.

"Some people have asked whether our Brexit deal is just a starting point from which we will regress," she said.

"Let me be clear. Our Brexit deal is not some long wish-list from which negotiators get to pick and choose. It is a complete plan with a set of outcomes that are non-negotiable."

Sorry, this video has expired Donald Trump backtracks on Brexit comments during joint press conference with Theresa May.

'Sue the EU'

Later, speaking to the BBC, Ms May said Mr Trump had previously advised her to sue the European Union as part of her Brexit strategy, revealing a piece of advice Mr Trump said on Friday she had ignored for being too "brutal".

"He told me I should sue the EU," Ms May told BBC television.

"Sue the EU. Not go into negotiations — sue them."

Ms May added that if party members called for a leadership spill she would stand in the contest, claiming that she is in it for the long term.

Ms May does not yet have a Brexit deal with the EU, so the British Government has stepped up planning for a so-called "no deal" Brexit that could spook financial markets and dislocate trade flows across Europe and beyond.

Ms May has repeatedly said Brexit would happen and has ruled out a re-run of the 2016 referendum, although French President Emmanuel Macron and billionaire investor George Soros have suggested that Britain could still change its mind.

David Davis, who resigned as Brexit Secretary last week, said it was an "astonishingly dishonest claim" to say there was no worked-out alternative to Ms May's plan.

Writing in the Sunday Times, he said her plan would allow EU regulations to harm British manufacturers.

"Be in no doubt: under the Government's proposal our fingers would still be caught in this mangle and the EU would use it ruthlessly to punish us for leaving and handicap our future competitiveness," Mr Davis said.

Trump says he spoke to Queen on Brexit

Sorry, this video has expired The Queen was forced to walk around Mr Trump after he stopped in front of her.

Before leaving Britain for a summit in Finland with Russian President Vladimir Putin, Mr Trump breached British Royal protocol by publicly disclosing the details of a conversation he had with Queen Elizabeth about the complexities of Brexit.

When asked if he discussed Brexit with the monarch as they met for tea at Windsor Castle on Friday, Mr Trump said:

"I did. She said it's a very — and she's right — it's a very complex problem, I think nobody had any idea how complex that was going to be.

"Everyone thought it was going to be, 'Oh it's simple, we join or don't join, or let's see what happens'."

Mr Trump and the Queen inspect a Guard of Honour at Windsor Castle. ( AP: Matt Dunham/Pool )

Speaking of the Queen, Mr Trump was quoted as saying: "She is an incredible woman, she is so sharp, she is so beautiful, when I say beautiful — inside and out. That is a beautiful woman."

Asked if he felt the Queen had liked him, he said: "Well I don't want to speak for her, but I can tell you I liked her. So usually that helps. But I liked her a lot.

"Just very elegant. And very beautiful. It was really something special.

"She is so sharp, so wise, so beautiful. Up close, you see she's so beautiful.

"She's a very special person."

Mr Trump also made clear his admiration for former foreign minister Boris Johnson, who Mr Trump said would one day make a great British prime minister.

Steve Bannon, Mr Trump's former adviser, was even quoted by Britain's Daily Telegraph as saying that it was now time for Mr Johnson to challenge Ms May for her job.

"Now is the moment," The Telegraph quoted Mr Bannon, Trump's former strategist and a key player in his 2016 election campaign, as saying.

"If Boris Johnson looks at this … there comes an inflection point, the Chequers deal was an inflection point, we will have to see what happens," Mr Bannon said.

Reuters