British Prime Minister Theresa May warned her divided party on Sunday that there may be "no Brexit at all" if they wrecked her plan to forge a close relationship with the European Union after leaving the world's biggest trading bloc.

"My message to the country this weekend is simple: we need to keep our eyes on the prize," May wrote on Facebook. "If we don't, we risk ending up with no Brexit at all."

Linking the fate of Brexit to her own survival in such an explicit way indicates just how precarious May's position remains after her government was thrust into crisis and U.S. President Donald Trump publicly criticised her Brexit strategy.

May is fighting off the right of her own party which is pushing for a “hard Brexit” and sees her as weak on the issue. May's position was further undermined by Trump who said in an interview published in Rupert Murdoch's Sun newspaper on Friday that her proposals would probably kill off any chance of a post-Brexit trade deal with the world's biggest economy.

May on Sunday revealed that Trump had previously advised her to sue the EU rather than enter into an exit negotiation with the bloc.

"He told me I should sue the EU," she told BBC television. "Sue the EU. Not go into negotiations - sue them."

Though Trump later contradicted his comments by then promising a great U.S. trade deal, the president made clear his admiration for the 54-year-old Boris Johnson, who Trump said would one day make a great British prime minister.

Steve Bannon, Trump's former adviser, told Nigel Farage on LBC radio Sunday: "You're going to have to fight to take your country back, every day. Whether it's Italy, France, England, or the United States. If we quit, they're going to be in control."

Theo Usherwood, LBC's political editor, said: "That sounds like a call to arms.” To which Bannon replied, "Absolutely. This is war.”

Bannon was also quoted by Britain's Daily Telegraph as saying that it was now time for Johnson to challenge May for her job.

"Now is the moment," The Telegraph quoted 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," Bannon said.

With less than nine months to go before the United Kingdom is due to leave the EU on March 29, 2019, the country, the political elite and business leaders are still deeply divided over what form Brexit should take.

By warning that Brexit itself is in danger, May is sending a blunt message to the dozens of hardline Brexiteers in her party that if they sink her premiership then they risk squandering the victory of an EU exit that they have dreamed about for decades.

Some pro-Brexit Conservatives fear that a deal could emerge that leaves Britain tightly bound to EU rules and represents a Brexit in name only.

The British government has also stepped up planning for a so called "no deal" Brexit that could spook financial markets and dislocate trade flows across Europe and beyond.

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

Seeking to strike a balance between those who want a smooth Brexit and those who fear staying too close to the EU's orbit, May sought the approval of senior ministers for her plans on July 6.

After hours of talks at her Chequers country residence she appeared to have won over her cabinet, but just two days later David Davis resigned as Brexit secretary, followed by her foreign minister, Boris Johnson, the next day.

May called on Sunday for the country to back her plan for "friction-free movement of goods", saying it was the only option to avoid undermining the peace in Northern Ireland and preserving the unity of the United Kingdom.

Davis, writing in the Sunday Times, said it was an "astonishingly dishonest claim" to say there is no worked-out alternative to May's plan. 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," Davis said.

Steve Baker, a senior lawmaker who served as a deputy to Davis in the Brexit ministry before resigning with his boss, said May had presided over a "cloak and dagger" plot to undermine Brexit.

TENSIONS IN PARLIAMENT

Johnson, the face of the Brexit campaign for many has remained silent in public since he warned in his resignation letter on July 9 that the "Brexit dream" was being suffocated by needless self-doubt.

When asked on Sunday if she would stand if a Conservative Party leadership contest was triggered, May declined to directly answer saying: "I am in this for the long term."

The extent of the danger to May from rebels in her party will become clearer over the course of two debates in parliament this week.

Pro-Brexit lawmakers are expected to use a debate on Monday on customs legislation to try to force her to harden up her Brexit plan, while a debate on trade on Tuesday will see pro-EU lawmakers push for even closer ties with the bloc.

Brexiteer rebels are unlikely to have enough support in parliament to win a vote, but the debate will show how many in May's party are prepared to vote against her at a time when some are looking to gather the numbers needed to challenge her leadership.

If May does clinch a Brexit deal with EU leaders, lawmakers could still vote it down.

Voting down a divorce deal so late in the process would trigger a major political crisis in Britain which would then be on course to crash out without any formal Brexit arrangements for trade, immigration and security.