Pensions secretary Esther McVey pointedly refused to endorse the PM's plan during the latest cabinet meeting and global development secretary Penny Mordaunt and the leader of the Commons Andrea Leadsom also made it clear they held deep concerns.
Negotiations in Brussels have stepped up in recent days ahead of a high-stakes European Union summit next week, with both sides seeking a breakthrough less than six months before Brexit in March 2019.
Britain rejects the EU's proposed solution - to keep Northern Ireland inside the bloc's single market and customs union after the rest of the UK leaves - because it would create new barriers between Northern Ireland and the rest of the UK The EU has asked for a counter-proposal from Britain.
And the BBC understands another senior minister, Commons leader Andrea Leadsom, was "thinking carefully about whether she could put up with such a compromise".
Brexit Secretary Dominic Raab has said he is confident Britain would be able to negotiate such agreements with the likes of France, Belgium, and the Netherlands.
Political blog Guido Fawkes this morning cited sources as saying the PM will today make a public statement to scotch concerns Britain will be "trapped permanently in a customs union in any circumstances". Her spokeswoman said the meeting on Thursday was one of a number held with groups of key ministers.
The pledge came amid speculation over possible ministerial resignations if the Prime Minister gives too much ground ahead of the Brussels summit next week.
But the United Kingdom says that would create a border between Northern Ireland and Great Britain and is unacceptable.
"Despite all the so-called no deal planning, the Government has yet to admit that a no deal would require a raft of substantial legislation to be rushed through Parliament, crucial stop gap agreements with the European Union on matters relating to Northern Ireland and security, and the recruitment of thousands of custom officials".
May hopes British negotiators can strike a deal over the weekend, which she can discuss with the cabinet on Tuesday.
British finance minister Philip Hammond said there had been a "measurable change in pace" in the talks, but there were "some big differences left to resolve".
If a deal is done, he suggested Britain could enjoy a "deal dividend" which "should deliver us an upside in the form of higher economic growth and better outcomes".
London published its latest round of documents planning for a "no deal" Brexit scenario Friday, including warnings that Northern Ireland could face electricity supply disruptions and hinting at problems with Eurostar cross-Channel trains.