The European Union is poised to extend Brexit talks into as late as next summer after the European council and commission presidents dismissed Boris Johnson’s strategy as a “blame game”.

A “range of dates” will now be in play at the meeting of European leaders next week but sources suggested the natural cut-off date would be June.

With an extension of the UK’s EU membership now looking inevitable, other diplomatic sources suggested an unlikely outlier for an end date could even be ahead of a possible general election so as to force the Commons into accepting a deal. “But politicians like to keep things off their plates for as long as possible and so pushing it longer seems more realistic,” a senior EU diplomat said.

The negotiations over a deal are said to be effectively dead in Brussels after Downing Street’s extraordinary claims over the substance of a phone call between the German chancellor, Angela Merkel, and the UK prime minister.

Merkel was said by an unnamed UK source to have told Johnson that Northern Ireland had to stay in the EU’s customs union. The official claimed that as a consequence a deal looked “essentially impossible, not just now but ever”.

Tusk, the European council president, gave an insight into the frustration at the anonymous briefings over the Merkel call, the alleged content of which described by senior politicians in Berlin as “improbable”. The chancellor’s spokesman declined to comment on “confidential conversations”.

“What’s at stake is not winning some stupid blame game,” Tusk wrote in a tweet directed at the prime minister. “At stake is the future of Europe and the UK as well as the security and interests of our people. You don’t want a deal, you don’t want an extension, you don’t want to revoke. Quo vadis? [Where are you going?]”

The European commission president, Jean-Claude Juncker, said: “I do not accept this ‘blame game’ of pinning the eventual failure of the negotiations on the EU.

“If that’s the case the explanation is actually in the British camp because the original sin is on the islands and not on the continent.

“Nobody would come out a winner in this scenario. A no-deal Brexit would lead to a decline of the UK and a clear weakening of the roots of growth on the continent.”

Juncker said that Johnson’s Brexit proposals would leave the UK with a relationship with the EU that was “less intimate than with Canada”.

In Berlin Detlef Seif, the point person on Brexit for Merkel’s party, the CDU, rejected the account given by Downing Street of the call between the two leaders.

He said: “In my mind it is completely improbable that the phone call between Merkel and Johnson took place in the way it has been reported in the British media.

“It would run counter to all the principles the German government has followed for the last three years, namely that the negotiations are led by the European commission.

“For the German chancellor to insist on Northern Ireland remaining in the customs union would completely breach these guidelines.”

Germany has been one of the most outspoken advocates for allowing the UK as much flexibility as possible to avoid a no-deal scenario on 31 October or at a later date.

The unnamed Downing Street briefings have been widely attributed to Dominic Cummings, the former Vote Leave supremo who is now the prime minister’s chief adviser.

“There has been a lot of scepticism about Johnson’s proposal in Berlin, but Merkel’s attitude has always been a positive one, to find out if there is room for a compromise,” Sief said.

“The only explanation I can see for these reports is that Johnson is trying to build a story where he blames Germany for a no-deal Brexit. To brief out a confidential phone call in such a manner is utterly unprofessional and infuriating to anyone who has been working on a deal”.

On Tuesday, Johnson spoke to the Irish taoiseach, Leo Varadkar, in a 40 minute call and agreed to meet later this week in what will be seen as a last-ditch chance to find a deal.

The briefings attributed to Cummings had suggested that Varadkar had gone back on his word by attacking Johnson’s proposals for the Irish border, which involve a customs border on the island of Ireland and Northern Ireland staying in the single market for goods.

In Brussels, thinking is moving to how to react to what is seen as an inevitable request for an extension of the UK’s EU membership.

A debate among EU27 states over the end date for any extension is yet to take place.

Following a meeting with Johnson in No 10 on Tuesday, the European parliament’s president, David Sassoli, said there had been “no progress” in talks but MEPs were open to a Brexit extension.

Johnson has repeatedly said that he will not comply with the Benn act, which would instruct him to request an extension by 19 October if a deal is not secured with the EU. He has also insisted that the UK will leave on 31 October with or without a deal.

The unnamed source had suggested to the Spectator magazine on Monday night that the UK would be a truculent member, blocking the EU’s plans.

But the only key issue on which the UK could wield its veto would be the EU’s seven-year budget which is unlikely to come to a vote until June, or September at the latest. “We could extend to September without any problem, but what would you achieve over the summer? So June seems more likely,” said an EU diplomat.

“Beyond the UK being able to veto a budget, an end date by then is important because we don’t want the UK to be in and net recipients to be able to argue that there is no reason for a cut to the budget.”

The Benn act suggests an extension until the end of January 2020 but there will be some concern in EU capitals that this will not provide sufficient time for any fallout from a general election to play out.