The spin from Downing Street had been that Theresa May’s meeting with her Irish counterpart, Leo Varadkar, shortly after breakfast in the margins of an EU summit in Austria, had been “relatively warm”, albeit “frank”. The dawning truth later that evening was that, in a premiership littered with missteps, May had made one of her worst errors of judgment as the two leaders and their teams met in a private room in Salzburg’s Mozarteum University.

For weeks the working assumption in Brussels had been that, on the Irish issue at least, a major step forward would be made by the next leaders’ summit in October. But over the coffee the prime minister dropped a bombshell. She did not believe it would be possible for the British government and Brussels to come to a solution by then. Six months after promising to come up with a fix that would avoid a hard border on the island of Ireland in all possible circumstances, the British appeared to be stalling for time again.

The message reverberated around the Salzburg summit and reached the ear of the French president, Emmanuel Macron.

The intention had been that this would be a good summit for the prime minister, giving her something to work with on the eve of a difficult Conservative party conference. “Things didn’t happen as we expected,” an EU official admitted.

The French president ripped up the plan to offer Theresa May warm words along with an extraordinary Brexit summit on 17 and 18 November in order to finalise the terms of a Brexit deal. During a two-hour Brexit discussion over lunch among the EU27 heads of state and government, Macron told his fellow leaders that the prime minister should not be allowed to drag her heels. The pressure for a result needed to be increased.

May was to be set a threshold that she would have to reach if she wanted a deal. The EU’s leaders were instructed to increase their preparations for a no-deal Brexit. Viktor Orbán, the populist Hungarian prime minister, who had bowed and kissed May’s hand the previous evening before dinner, and boasted to reporters on Thursday of being part of a growing camp of leaders opposed to “punishing the British”, did not demur. “He did not say a word,” said a source.

After informing May of the developments in a brief and cursory meeting, Donald Tusk, the European council president, who did not appear happy in his task, informed reporters at a press conference at the end of the day: “The moment of truth for Brexit negotiations will be the October European council. In October we expect maximum progress and results in the Brexit talks. Then we will decide whether conditions are there to call an extraordinary summit in November to finalise and formalise the deal.”

The European commission president, Jean-Claude Juncker, said: “A no deal is not my working assumption but would it happen we are prepared … Don’t worry, be happy.”

A senior EU official, who could not rise to Juncker’s challenge, lamented: “This makes it more difficult. Macron and others wanted a higher degree of pressure in the negotiations and more uncertainty. But this does make things more difficult.”

The morning mistake from May, was then followed, it appears, by a misjudgment by the EU’s leaders. Along with Tusk’s ultimatum, came a kick, perhaps one that will prove fatal, at the Chequers proposals.

Downing Street had taken some hope, understandably, from Tusk’s words the previous day when he had said that Chequers was indicative of an “evolution” in British thinking. In return, it was suggested the EU would evolve its position. Now Tusk said of the leaders’ Brexit discussions: “Everybody shared the view that while there are positive elements in the Chequers proposal, the suggested framework for economic cooperation will not work. Not least because it risks undermining the single market.”

EU officials and diplomats insisted shortly afterwards that the choice of language was not designed to scupper May. “On the indivisibility of the four freedoms, and the integrity of the single market, the position has not changed and there is absolute unity,” said one.

A second said that the very fact that Tusk and others had not dismissed Chequers in its entirety was an example of the EU being “nice”. After all, the official said, all the prime minister had done over dinner the evening before during a much-hyped appeal to the EU for compromise was to “read out an op-ed” she had written for that morning’s Die Welt newspaper.

The strident tone of that article, published on the eve of the summit, was another cause of irritation among some, sources said.

One European minister said they had withheld their true feelings about the hopelessness of the prime minister’s plan.

But if Tusk’s words were not meant to hurt, Macron’s comments surely did have menace. “It was a good and brave step by the prime minister,” Macron said of Chequers. “But we all agreed on this today, the proposals in their current state are not acceptable, especially on the economic side of it.” He said of the suggestion from May that the only alternative was a no-deal scenario: “The Chequers plan cannot be take it or leave it.”

With Orbán’s thinly veiled attack on his tough approach to the UK no doubt in mind, Macron added: “Those who explain that we can easily live without Europe, that everything is going to be all right, and that it’s going to bring a lot of money home are liars. It’s even more true since they left the day after so as not to have to deal with it.”