The DUP has warned Theresa May to “keep her side of the bargain” on Brexit after the party’s 10 MPs withheld their support in votes on Budget measures.
Mrs May insists her draft EU withdrawal deal, which MPs will vote on next month, is the right one.
She remains under pressure from Tory MPs to seek concessions from the EU.
The DUP, which props up the government, said it was sending the PM a “message” over her plan to “separate” Northern Ireland from the rest of the UK.
The party’s decision to abstain on a series of amendments to the Finance Bill on Monday night – as well as support an amendment proposed by Labour – was seen as a gesture, rather than an attempt to influence policy.
European leaders are set to sign off the draft withdrawal agreement and framework of future relations in Brussels next month.
Mrs May’s new-look cabinet meets on Tuesday for the first time since a number of ministers resigned last week, after they said they could not support the deal.
But the prime minister continues to be the target of backbench critics within her party, who appear intent on forcing a confidence vote on her leadership.
If the chairman of the 1922 Committee of backbench Conservatives – Sir Graham Brady – receives letters from 15% of the party’s MPs, a secret ballot is triggered.
A source has told the BBC they have not given up hope of reaching the required threshold of 48 letters.
Not everyone who had pledged to send in letters has done so, the source said. As of Monday evening, only 26 had publicly confirmed they had.
Ministers from the remaining 27 EU countries have met in Brussels to work on the political declaration setting out their future relationship with the UK.
Both the UK and EU have indicated that the draft withdrawal agreement reached last week – including the controversial “backstop” plan to avoid the return of customs posts on the Irish border in the event no new trade deal is in place – is unlikely to be changed.
But the BBC’s Laura Kuenssberg has been told by senior Brexiteers that discussions with Number 10 earlier were “constructive”, while a minister also said to “expect a little movement”.
As it stands, there appears to be a majority in Parliament against the deal.
The Democratic Unionist Party has made it clear it cannot support the agreement as it stands, saying the backstop would result in new regulatory barriers between Northern Ireland and the UK.
The party has been propping up Mrs May’s government in key votes in the House of Commons since last year’s inconclusive election result, via a “confidence and supply” arrangement.
But on Monday its MPs abstained on a series of votes on the Finance Bill, legislation needed to implement measures in last month’s Budget, and voted with Labour on one occasion to cut the government’s majority to just five.
It sent a “political message” to the PM over her willingness to “separate” Northern Ireland from the rest of the UK, Brexit spokesman Sammy Wilson said.