Theresa May was forced to insist her Chequers plan was not dead and accused European Union leaders of engaging in “negotiating tactics” after she was ambushed at the end of Salzburg summit when Donald Tusk declared that her proposals would not work.

Brexit: May says Chequers 'only plan' on table after EU calls it unacceptable - Politics live Read more

The prime minister acknowledged that she had had a “frank bilateral” with Tusk an hour before her end-of-summit press conference in which she was forced to defend Chequers and maintain that it was possible to reach a Brexit deal in the autumn.

May had met Tusk after the other EU leaders had discussed Brexit in her absence over lunch, amid signs that they were attempting to harden their stance. But she tried to downplay their unexpectedly strong criticisms of her Chequers plan.

“I have always said these negotiations were going to be tough,” May told reporters. “And at various stages of these negotiations, tactics would be used as part of those negotiations.”

May had hoped that EU leaders would give her Chequers plan a warm reception – or at least not actively criticise it – 10 days before she has to face a restive Conservative party at its annual conference next month.

But the response of Tusk left May on the defensive, and boosted her political critics at home. “Chequers goes pop,” declared Jacob Rees-Mogg, the chairman of the hard Brexit ERG group, on twitter and he later argued that the prime minister now needed to abandon her plan and instead pursue “a Canada plus free trade deal”.

Tusk had forewarned her in their lunchtime meeting, giving her an hour to prepare for a press conference in which she appeared nervous and angry, where she was forced to repeat that her proposals could still form the basis of future relationship negotiations at the EU’s October summit and a likely emergency summit in November.

Donald Tusk demands answer to Irish border question next month Read more

May repeatedly defended Chequers, which proposes that the UK share a common rulebook for goods and services after Brexit, because that would help ensure that trade could flow freely across the Irish border. “Our white paper remains the only serious and credible proposal on the table for achieving that objective,” she said.

The prime minister also said she would presenter a counter-proposal to resolve the impasse over the Irish backstop, an EU proposal stipulating that if a free trade agreement cannot be signed after Brexit then Northern Ireland would remain in the customs union, creating a trade border down the Irish Sea. But she would not say when the proposals would be made public.

The British PM tried to shift the burden for future concessions on to her EU counterparts, saying a deal could be reached “if political will is there on the other side”.

The French president, Emmanuel Macron, said of Chequers: “We all agreed on this today, the proposals in their current state are not acceptable.”

Critics on the right of the Conservative party repeated their concerns about Chequers before the summit ended. David Davis, the former Brexit secretary, said in a speech on Thursday lunchtime that the EU “will demand more concessions” beyond Chequers to get a deal.

“Concessions on free movement, financial contributions and ECJ jurisdiction. The final deal is likely to look an awful lot like membership,” Davis claimed.