Theresa May has been humiliated by EU leaders who unanimously rejected her plans for Brexit.

European Council President Donald Tusk openly mocked her on social media.

Conservative MPs from all sides are now rounding on May in order to force her to drop her so-called Chequers plan.

The row comes ahead of an already difficult Conservative Party conference for the prime minister.

LONDON — Theresa May is in the midst of a serious crisis for her leadership after European leaders roundly demolished the Chequers plan she had spent the entire summer selling as the only credible plan for Brexit.

Donald Tusk, European Council President, on Thursday said May's idea that the UK could stay in the single market for goods after Brexit "will not work," and had been rejected by leaders of all 27 EU member states.

He later openly mocked her on Instagram, with a satirical post referring to the EU's rejection of her plans to "cherry pick" parts of the European single market. He was joined by French president Emmanuel Macron, who dismissed the entire Brexit project as being pursued by "liars".

It was a humiliating day for May, who had travelled to Salzburg, Austria quietly confident of receiving encouraging feedback on her Chequers proposals, after recent reports suggested Brussels was ready to soften its Brexit position.

However, what she received from her European counterparts was even more brutal than even the most pessimistic observers had expected, as reflected in this morning's front pages:

And it's not just the British press who are circling around May.

MPs from all sides of the Brexit divide are ganging up on the embattled prime minister and her corpse Brexit plan.

Cabinet ministers Sajid Javid, Andrea Leadsom, Penny Mordaunt, and Esther McVey all reportedly want May to forget Chequers and focus on a Canada-style free trade deal, while former minister Philip Lee last night called for the Article 50 negotiation process to be extended. His Remain-supporting Conservative colleague Anna Soubry compared May's Brexit plan to a dead parrot, while Iain Duncan Smith, a leading pro-Brexit MP, said that Chequers "clearly isn't going to fly." Meanwhile, Sir Keir Starmer, Labour's Brexit spokesperson, urged May to ditch her "her reckless red lines."

So where did it all go wrong?

It's no surprise that the EU rejected May's Chequers plan. Michel Barnier, the bloc's chief Brexit negotiator, said on numerous occasions this summer that its core proposals — the UK effectively stays in the single market for goods and collects EU tariffs on Brussel's behalf — were never going to be accepted.

A senior EU source told Business Insider as early as June that May's suggestions would not fly.

But 10 Downing Street was braced for a polite "not quite," accompanied by a "but we can make it work." It was not prepared for such a bruising slap down. It certainly was not prepared for French leader Macron to describe Brexit-supporting politicians as "liars" in a pretty extraordinary press conference.

—People's Vote UK (@peoplesvote_uk) September 20, 2018

Clearly, May's advisors — who she relies upon heavily for intelligence and guidance — had misread the mood.

Plus, according to multiple reports, May really did not help her cause when she sat down for dinner with European leaders.

May reportedly decided to read out an opinion piece she wrote for German newspaper Die Welt this week, in which she described the EU's Brexit proposals as unacceptable and not credible. That didn't go down well.

"She began to read out the article,” a diplomat told The Times. "The article they had all already read. That is not the way to command a tired group of leaders who are all a bit sick of each other."

The prime minister also opted to flat out reject the EU's updated proposals on how to deal with the thorny issue of the Irish border, which also didn't go down well with leaders around the table.

May also reportedly hadn't been helped by a BI story on Thursday that revealed Liam Fox, her trade secretary, was quietly plotting to use controversial government powers to scrap EU food standards in preparation for a trade deal with the US.

Macron and other European leaders raised the article over the dinner, according to The Times and other reports.