Donald Tusk has put a no-deal Brexit back on the table by saying EU leaders will only agree to a short delay if MPs back Theresa May’s deal next week, on a day of high drama in Brussels and London.

After belatedly receiving the prime minister’s formal letter requesting a three-month extension of article 50, and taking a late afternoon phone call with her, the European council president admitted that success appeared “frail, even illusory” on the eve of Thursday’s summit.

The German foreign minister, Heiko Maas, tweeted: “The letter from Theresa May has not solved any problem yet. If the European council [summit of leaders] is to decide on an extension of the deadline for Britain, we would like to know what is the concrete purpose.”

But Tusk said the EU would seek until the very last moment to avoid the UK crashing out without a deal and show “patience and goodwill” despite the “Brexit fatigue” in the capitals.

The EU27’s heads of state or government would be likely to agree in principle at the summit on Thursday to an extension up to 23 May or 30 June, and sign it off without needing to meet next week should May be able to find a majority in the Commons at the third time of asking, he said.

The European commission is insisting that an extension beyond the date of the European elections on 23 May would require British MEPs to be elected, although others believe there is little risk as long as the UK has left by 1 July when the parliament formally convenes.

“In the light of the consultations that I have conducted over the past days I believe that a short extension will be possible but to be conditional on a positive vote on the withdrawal agreement in the House of Commons,” Tusk said.

“A question remains open as to the duration of such an extension. Prime Minister May’s proposal of the 30 June, which has its merits, creates a series of questions of a legal and political nature. Leaders will discuss this tomorrow.”

Echoing the comments of Jean-Claude Juncker, his counterpart in the European commission, earlier in the day, Tusk added he would bring the leaders back for an “extraordinary summit” if May’s deal was again rejected by the Commons.

Brussels expects the British government to request a lengthy extension, and hold European elections, if the deal fails in the Commons at the third time of asking, so as to allow time for cross-party talks on a soft Brexit or a general election or second referendum.

Tusk’s comments came at the end of a day during which it appeared at one point that there would not be a letter of request at all from Downing Street. The contents when it did arrive came as a surprise.

EU diplomats had been led to believe by May’s de facto deputy, David Lidington, who was in Brussels on Tuesday, that the British government would be seeking a lengthy extension with an option to leave after three months should the deal pass.

After a stormy cabinet meeting, however, a letter from Downing Street did not appear overnight as had been expected. When it did emerge there was a sole request of a extension until 30 June to provide time for withdrawal legislation to be passed should the main deal be backed.

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The frustration and “tension”, as one senior EU diplomat described it, was made clear by the French foreign minister, Jean-Yves Le Drian, who told the French parliament on Wednesday afternoon that Paris was willing to block an extension.

He said there were only two ways to leave the EU: ratify the withdrawal agreement or a no-deal exit. If parliament did not ratify the withdrawal agreement “the central scenario is a no-deal exit”, he said, adding: “We’re ready.”

He stated that if May could not present “sufficient guarantees of the credibility of her strategy” that would lead to the extension being refused and a no-deal exit.

A senior diplomat in Brussels added of May’s request: “We now have a letter in front of us of which we are not sure the content has democratic support in the UK.”

A second EU diplomat said: “This strategy of delay, prolongation, irresponsibility and the inability to work across the chamber will be how I answer when asked by my mother and friends how I contributed to this mess.”

May will address the EU’s leaders on Thursday afternoon, before they deliberate without her on how to respond.

In Dublin, the Irish PM, Leo Varadkar, urged the EU to cut the British government “some slack” in its attempt to get a Brexit deal through parliament.

“There is a real risk of no deal happening by accident”, he said. “It’s time now to cut the British government some slack when it comes to their request for an extension.”