Theresa May has been urged not to allow Eurosceptic MPs in her party to "impose their own conditions" on negotiations amid signs of fresh Tory infighting.
Nineteen Tory MPs who back a "soft Brexit" have written to her saying it is "highly irresponsible" for anyone to dictate terms which may scupper a deal.
It follows some Tories backing the DUP's decision to oppose a draft deal on the future of the Irish border.
The PM has spoken to the DUP's Arlene Foster to try to break the deadlock.
The DUP says there is "more work to be done" if it is to agree to plans for the future of the border between Northern Ireland and the Irish Republic after Brexit - a prerequisite for talks to move on to their next phase.
And the BBC understands the ambassadors of the 27 EU member states, who received an update from chief EU negotiator Michel Barnier on Wednesday, are "waiting for something from London" in the next 48 hours.
The BBC's Adam Fleming said Mr Barnier and the member states agreed there must be clarity within 48 hours for them to have enough time to consult with their capitals about draft guidelines for phase two of the talks.
At a summit next week, European leaders will decide whether enough progress has been made in the negotiations on Ireland, the UK's "divorce bill" and citizens' rights so far to open trade talks.
In their letter, the 19 MPs - who largely backed Remain in the 2016 referendum - say they support the PM's handling of the negotiations, in particular the "political and practical difficulties" relating to the Irish border.
But they hit out at what they say are attempts by some in their party to paint a no-deal scenario in which the UK failed to agree a trade agreement as "some status quo which the UK simply opts to adopt".