EU leaders are preparing to hold an extraordinary “no deal” Brexit summit in November to deal with the potential consequences of the UK crashing out of the bloc should Theresa May fail to deliver decisive progress on the Irish border issue this week, the Guardian can reveal.

A special meeting of heads of state and government at which the EU had hoped to sign off on the Brexit negotiations next month may instead be turned into a emergency summit to discuss the bloc’s response to a cliff-edge Brexit.

The plan is likely to pile further pressure on the British prime minister by illustrating the EU’s seriousness about allowing the UK to crash out if the alternative were a deal that would undermine the integrity of the single market or prove unacceptable to the Republic of Ireland.

The European council president, Donald Tusk, told May last month that he needed to see “maximum progress” by this week’s European council meeting of leaders on the issue of avoiding a hard border on the island of Ireland.

A Brexit summit to finalise the terms of the political declaration on a future trade relationship has been pencilled in for the weekend of 17-18 November, in the event the negotiating teams find a compromise position on avoiding a hard border. EU sources said they had expected the summit to be a sombre ceremonial event.

In response to concerns over May’s ability to hold her government together and push through a deal, however, the EU is now planning an alternative use for the November summit should it be required.

The bloc’s deputy chief negotiator, Sabine Weyand, had told EU ambassadors gathering in Luxembourg on Friday that talks were progressing well, and that results might be made public as early as Monday. One senior EU diplomat said: “The wedding knot is tied.”

The two negotiating teams were due to make an assessment of the state of play on Sunday evening. A leaked EU planning document, obtained by the German newspaper Süddeutsche Zeitung, noted: “Deal made, nothing made public (in theory).”

May’s volatile domestic situation remains the greatest risk to a deal. Internal government emails leaked to the Observer revealed on Sunday that the DUP leader, Arlene Foster, had privately let it be known that following a “hostile and difficult” meeting with the EU’s chief negotiator, Michel Barnier, last week that she was now ready for a no-deal scenario. She said she regarded it as the “likeliest outcome”.

The former Brexit secretary David Davis for his part has appealed to the cabinet to ‘“exert its collective authority” to kill off May’s plans to keep the UK indefinitely in a customs union to solve the problem of avoiding a hard border on the island of Ireland.

The European commission’s secretary general, Martin Selmayr, told member states last month that their governments would need to decide how far it would be in the EU’s interest to mitigate some of the impacts of a scenario in which the UK leaves the bloc without a deal.

A senior EU diplomat said leaders at the November summit would also want to coordinate their responses in areas where national governments have competence, such as contingency measures to avoid long queues of lorries waiting at customs, or in aviation and haulage.

A senior EU diplomat said: “Preparations on contingency are really advancing in almost all member states. We’re in close contact with our neighbours and exchanging all these issues. The commission has beefed up its team working on contingency.”

“This is a parallel track. We’re going to do this anyhow whatever the outcome because even if there’s a positive outcome [this week] we’ll still need to continue preparedness and contingency because we can never exclude the possibility that negotiations will break down at a later stage”, the same source said.

“Unless we have a final deal agreed upon by the House of Commons and the British parliamentary system we’ll carry on with our preparedness and contingency. That’s normal work.”

The UK and EU agreed in December, and again in March, that the withdrawal agreement would contain an Irish protocol in which a backstop solution for avoiding a hard border under any circumstances would be spelled out.

The EU has proposed that if a trade deal or bespoke technological solution was not at hand by the end of the transition period, Northern Ireland would in effect stay in the single market and customs union as the rest of the UK withdraws.

May has insisted this would involve the constitutional dislocation of the UK. She is instead proposing a temporary EU-UK customs union, and for Northern Ireland to stay in the single market, should that be agreed at a later date by Stormont.