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.
Irish PM Leo Varadkar, who also spoke to Mrs May on Wednesday, said he was willing to consider new proposals but as yet none had been forthcoming.
In a separate development, Chancellor Philip Hammond has suggested the UK could pay the so-called Brexit bill, regardless of whether or not there is a subsequent trade agreement with the EU.
He told MPs on the Commons Treasury Committee he found it “inconceivable” that the UK would “walk away” from its financial obligations.
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”.
“We wish to make it clear that we are disappointed yet again that some MPs and others seek to impose their own conditions on these negotiations,” they write.
“In particular, it is highly irresponsible to seek to dictate terms which could lead to the UK walking away from these negotiations.”
It urges the PM to “take whatever time is necessary” to get the next stage of negotiations right.
On Tuesday, former Tory leader Iain Duncan Smith argued the time was fast approaching for the UK to consider walking away from the talks if the EU did not allow negotiators to proceed to the next phase – in which future trade and security relations will take centre stage.
The suggestion of “regulatory alignment” between Northern Ireland and the European Union has also concerned some Eurosceptic Conservative MPs.
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.
On Monday Northern Ireland’s Democratic Unionist Party – whose support the PM needs to win key votes at Westminster – objected to draft plans drawn up by the UK and the EU.
The DUP said the proposals, which aimed to avoid a “hard border” by aligning regulations on both sides of the border between Northern Ireland and the Republic of Ireland, were not acceptable. Dublin says it wants firm guarantees that a hard border can be avoided.
This has left the UK government racing to find an agreement suiting all sides in time for next week’s summit.
Following Mrs May’s call to Ms Foster, a DUP spokesman said discussions were continuing and there was “more work to be done” but the DUP’s deputy leader Nigel Dodds said the Irish government was playing a “dangerous game” with its own economy.
Speaking on Wednesday, Irish PM Leo Varadkar said he was willing to consider any new proposals from the UK over the border issue and suggested a resolution could possibly be put off until the New year.
On the issue of the divorce bill, a No 10 spokesman said the government’s position remained that “nothing is agreed until everything is agreed and that applies to the financial settlement”.
Mr Hammond had suggested it would not be “credible” for the UK to leave without honouring its obligations as “frankly it would not make us a credible partner for future international agreements”.
Reports have suggested the UK has raised its financial offer to a figure of up to 50bn euros (£44bn).