The prime minister is facing threats from all sides, with the DUP adding to her list of headaches by firing a “warning shot” hours after she said she was “determined” to deliver Brexit.
The Northern Irish party flexed its muscles by refusing to support a crucial government finance law.
It effectively breached the confidence and supply agreement that keeps Mrs May’s government in power.
The deal binds 10 DUP MPs to “support the government on all motions of confidence; and on the Queen’s Speech; the budget; finance bills”.
A party source accused the prime minister of “acting cavalierly” and warned her not to expect “business as usual”.
They told Sky News: “The government have got to remember the confidence and supply agreement on the shared priorities of Brexit, which is control of borders, money and preservation of the union.
“They can’t act cavalierly and breach sections of the confidence and supply agreement and expect business as usual.”
The source added that their abstentions on amendments to the new law, which will introduce tax and duty changes announced in the budget, was intended as a “warning shot”.
Sky News’ Laura Bundock said: “This is as big a message as possible to Theresa May.”
The DUP have been highly critical of the draft divorce deal, which was published last week.
Chief whip Jeffrey Donaldson suggested earlier this month it was “not something we can support”.
Despite the pressure from the DUP and members of her own party piling on pressure by submitting “no confidence” letters, Mrs May has vowed to battle on.
She claimed that leaving the EU was “never going to be easy or straightforward”, particularly in the closing stages of negotiations.
“But we have in view a deal that will work for the UK,” Mrs May said in a speech at the Confederation of British Industry (CBI) conference.
“And let no one be in any doubt, I am determined to deliver it.”
According to Sky News’ tally, some 25 Conservatives MPs have said they have submitted letters calling for a confidence vote in the prime minister.
A total of 48 are needed, and the only person who knows the true number received is chair of the 1922 committee Sir Graham Brady.
Sky News political correspondent Lewis Goodall said: “If we do get through the next day or so and no letters appear, the European Research Group (ERG) will have succeeded in not diminishing the prime minister’s authority but enhancing it.”
The government also conceded to Labour MP Chuka Umunna’s bid to force it to publish analysis comparing the “costs and benefits” of current EU terms with its own plan, and a no-deal divorce.
Earlier this evening, senior Leave campaigning MPs headed to Downing Street for an hour-long meeting with Mrs May, to lobby her to change the draft deal before it is finalised.
Sky News’ deputy political editor Beth Rigby said the fact the group did not stop on their way out of Downing Street to speak to waiting media is being taken in some quarters as a sign the meeting may have been constructive.
She added: “There is a bit of a division now between one ERG Brexiteer group… and another group in the ERG that doesn’t think it’s the right time – and I’m told they would like to wait a bit to watch the prime minister’s withdrawal agreement bill fail in the House of Commons – and then trigger the vote of confidence.
“So some splits in the ERG as to when to put those letters in… it perhaps explains why we haven’t got the full number (of 48) quite yet.”
Ahead of a summit on Sunday where EU leaders are set to sign off on the withdrawal agreement, both London and Brussels have been at pains to point out that it cannot be renegotiated.
Mrs May said the draft plan had been “agreed in full” by both sides.