Britain’s new exit date from the EU, and the conditions attached to a Brexit delay, will likely be fixed in the gilded rooms of the Belgian prime minister’s 16th century Egmont Palace hours before Theresa May addresses the leaders.

Under emerging plans, a small group of EU leaders whose countries will be most affected by the UK’s departure will be hosted by the Belgian PM, Charles Michel, on Wednesday afternoon. The guest list is likely to include the leaders of France, Germany, Spain, the Netherlands, Denmark and Ireland.

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The purpose of the proposed coordinating meeting, three hours before May was set to address the full complement of 27 heads of state and government, would be to try to shepherd the debate that would be held later on, and avoid potentially catastrophic errors, the Guardian has learned.

The bombastic positions emerging from Paris over any sort of extension without a clear purpose – beyond a two-week buffer after 12 April to prepare the markets for no deal, along with the warning from Leo Varadkar, the Irish taoiseach, that a member state that vetoed one would “never be forgiven” – have injected peril into the process, sources said.

The Finnish foreign minister, Timo Soini, told reporters in the margins of a meeting of foreign ministers in Luxembourg that without a significant sign of progress in the cross-party talks embarked upon by May “it may be that an extension is not so easily to be achieved”.

Luxembourg’s foreign minister, Jean Asselborn, said: “The British have turned a deal into a no-deal. And now they want to turn the no-deal into a deal.”

Senior EU diplomats insisted, however, that it was inconceivable now that Britain would leave without a deal this Friday, or within the next couple of weeks. One diplomat said “France’s position is moving”.

The EU’s chief Brexit negotiator, Michel Barnier, has pledged support for the Irish border backstop regardless of what happens in the Brexit negotiations.

“The EU will stand fully behind Ireland,” Barnier said on Monday at a joint press conference with Varadkar in Dublin.

May has been seeking an extension up until 30 June to get her deal passed, while accepting she would need to hold European elections on 23 May if the UK was still a member state.

The foreign secretary, Jeremy Hunt, laid out the reasoning behind that position during his bilateral talks in Luxembourg on Monday with his Polish, Hungarian, Dutch and Portuguese counterparts.

The British prime minister was “leaving no stone unturned” in her cross-party talks with Jeremy Corbyn, Hunt insisted, 48 hours before a summit where the government was expected to show there was a purpose to such an extension.

The advantage in an extension to 30 June would lie in creating yet another cliff-edge moment to keep the pressure on MPs, although there was little doubt that if nothing was resolved by that point the EU would extend again.

The suggestion by Donald Tusk, the European council president, of a long extension of a year with the option for the UK to leave when the 585-page withdrawal agreement and 26-page political declaration was finally ratified has gained support among some member states.

That would have the advantage of the UK owning its fate, and the EU’s leaders avoiding having to repeatedly get involved in British politics when the extension renewal comes up, sources said.

Play Video 2:16 Theresa May warns that parliament must agree a deal to deliver Brexit – video

Those advocating for such an extension have suggested it would show a contemptuous attitude to European elections for the UK to have MEPs who “as a baseline” would be out of a job within two months of being elected. But the difference between the positions of an extension until 30 June, that could be repeated, or a longer delay, that could be shortened, was small, sources said.

The focus of attention for a number of EU states was to ensure the British government signed up to conditions that would bring “sincere cooperation” from the UK during its extended membership, including restraining from involvement in budget discussions or agreeing to go without a European commissioner.

The Spanish government was understood to have drafted a list of possible measures the UK would need to agree for Madrid to be convinced that an extension would not allow a future Brexiter prime minister to damage EU interests.

Additional reporting by Rory Carroll in Dublin