If Theresa May agrees to an Irish backstop that keeps the UK in the customs union indefinitely she is likely to face an outcry from Brexiters that could lead to resignations from her cabinet.

Sources close to leave-supporting ministers described the atmosphere as “tense” and “febrile” as they awaited the outcome of unexpected crunch talks in Brussels on Sunday between the Brexit secretary, Dominic Raab, and the EU’s chief negotiator, Michel Barnier.

Conservative party sources said May could survive a single resignation, but might not be able to do so if several ministers left at once. Here are the three who are considered most likely to quit.

Andrea Leadsom

The leader of the House of Commons will be a crucial figure if Theresa May’s proposals for Brexit are to make it through the House of Commons.

A former leadership candidate with some backbench support, she has little personal loyalty towards the prime minister after being demoted from the post of environment secretary after the general election. But she does not want to destroy either Brexit or a Conservative government with a futile gesture.

The South Northamptonshire MP angered some of May’s advisers earlier this month when she refused to back the prime minister if she chose to remain in No 10 once the UK had left the EU.

Those same advisers were furious when leaked minutes from the cabinet meeting that first discussed the Chequers plan showed she “hated” the proposals, which she saw as breaching the government’s red lines.

However, Leadsom is known to be cautious about resigning unless it will actually change the current direction of negotiations. In July, she criticised Boris Johnson’s resignation as foreign secretary, saying she still had faith that the UK under May would leave the EU the customs union and get rid of free movement.

Penny Mordaunt

The Brexiter – who with Leadsom, Johnson and Michael Gove appeared at the memorable Vote Leave press conference minutes after David Cameron resigned from office – has refused to back the Chequers plan.

The international development secretary is believed to be “deeply concerned” by reports that Britain could enter a deal with an indefinite arrangement to remain within the customs union.

She is being seen as a possible frontrunner to succeed May and will be aware that party members rarely support ambitious ministers who have deposed a sitting Tory prime minister.

A former navy reservist, she may feel some loyalty towards May who has promoted her twice. When May took office in 2016, she became minister for disabled people, before moving to her current post after Priti Patel resigned last year.

Esther McVey

The under-fire work and pensions secretary refused to back May’s Chequers plan this week while being interviewed by the BBC – repeatedly saying she was “fully, 100% behind the prime minister” without endorsing her proposal for future trade with the EU.

An arch-Brexiter and an ally of Iain Duncan Smith, she is said by colleagues to be concerned by the possibility of continued links with Brussels and plans for the Irish border.

McVey has been heavily criticised in recent weeks because of a succession of problems affecting universal credit, the government flagship welfare programme. She admitted last week that some claimants would be “worse off” – a response that contradicted the official government response – and declined to say whether the new benefits scheme would cost three million families £1,800 a year.