Theresa May has briefed senior ministers on the status of the Brexit negotiations as concerns grow in some quarters of her cabinet and party that she is prepared to agree an indefinite customs backstop to ensure an open border in Ireland.

The prime minister summoned members of the Brexit “war cabinet” on Thursday to build support for the terms of the withdrawal agreement and try to head off resignations before seeking the backing of her full cabinet and the rest of her party.

Ahead of the critical European summit next week, some cabinet members want any backstop arrangements – which could place all of the UK in a customs union after the end of the Brexit transition period in 2020 – to be clearly time-limited. But EU officials say that May has conceded that it will not contain a firm end date.

Instead, the UK would stay in a customs union, likely to be described as a customs arrangement by the prime minister, on an indefinite basis. However, the British government would be able to point to criteria under which the arrangement would be terminated, fulfilling the British desire for it to appear to be a temporary arrangement.

The nature of those criteria, and additional language to be used about the future deal, required by the UK to assuage concerns about the ultimate need for such a backstop solution to avoid a hard border on the island of Ireland, are still open to debate.

The Irish backstop forms part of the exit treaty that the UK will sign with the EU when it leaves; it spells out what customs and regulatory arrangements should operate in Great Britain and Northern Ireland in the event a comprehensive free trade deal cannot be signed by the two parties before the end of 2020 when the Brexit transition period ends. Hard Brexiters are concerned the UK could remain locked into the backstop arrangements indefinitely, particularly if it proves difficult to conclude a free trade deal.

The challenge for May will be in selling the formulation to the whole cabinet and avoiding resignations. Full cabinet members concerned about the idea of an indefinite backstop and considered at risk of quitting include Andrea Leadsom, the leader of the Commons who is very concerned about the lack of a time limit, and Esther McVey, the work and pensions secretary, who refused to specifically endorse May’s Chequers blueprint for Brexit earlier on Thursday – although she insisted that she was “completely supportive of the prime minister”.

McVey’s formulation echoed that used on Tuesday by Penny Mordaunt, the international development secretary, who said that while “the prime minister can count on my support” she did not know “where this is going to end up”.

Those present included Jeremy Hunt, the foreign secretary, David Lidington, the Cabinet Office minister, Sajid Javid, the home secretary, Michael Gove, the environment secretary, and Liam Fox, the international trade secretary. No 10 said the meeting was an informal one, meaning no decisions were taken; the key political meeting before next week’s summit will come on Tuesday at full cabinet, one day before the European council meeting begins.

Earlier, No 10 played down suggestions that the government was on the brink of unveiling a Brexit deal. “There are big issues still to resolve. That was the case on Monday and it remains the case today,” the prime minister’s official spokesman said.

The EU’s chief negotiator, Michel Barnier, told the European commission president and other commissioners in a private meeting on Wednesday that the principles of an agreement on the Irish border are largely accepted by both sides.

May is struggling to contain restive hard Brexiters in her own party, and an angry DUP, which fears that she has already made too many concessions in negotiations going on behind the scenes ahead of next week’s critical European summit in Brussels.

Shortly after the meeting began, the DUP leader, Arlene Foster, said that May and her cabinet colleagues who had told her they were unionists “could not in good conscience recommend a deal which places a trade barrier on United Kingdom businesses moving goods from one part of the kingdom to another”.

The unionist leader said she feared that the EU was forcing Northern Ireland into a deal that amounted to “the worst of one world” in which the region would remain part of the single market and remain under the ECJ, and would remain subject to trade restrictions with the rest of the UK.

A day earlier, the DUP had said it was prepared to vote down the budget if the prime minister in their eyes conceded too much ground to Brussels in the EU talks, risking an abrupt end to the confidence and supply agreement that has kept May in Downing Street since the 2017 election in which the Tories lost their overall majority.

Foster alluded to that threat, saying these were “significant days for Northern Ireland and the United Kingdom as we know it” and concluded by saying the DUP and its 10 MPs “will take decisions based on what is best for everyone in Northern Ireland and the rest of the United Kingdom.”

Michelle O’Neill, Sinn Fein’s vice-president, responded by saying that “the DUP does not speak for the majority of the people of the north on Brexit”. She said that the nationalist party supported remaining in the customs union and single market because it would give the north continued unfettered access to the EU market of around 500 million people and the British market of 67 million”.

Whitehall sources said before the meeting that they expected that key members of the cabinet would also discuss how to handle the DUP. They suggested that senior ministers including Lidington, May’s de facto deputy, Northern Ireland secretary Karen Bradley and chief whip Julian Smith would try to find a solution to the DUP’s demands. “They’ll be discussing what we need to offer the DUP and how to get them on side,” one said.