Theresa May is facing a fresh attempt by a cross-party groups of MPs to prevent a no-deal Brexit if she fails to reach an agreement with Brussels by mid-March.
Labour MP Yvette Cooper and Tory former minister Sir Oliver Letwin are among the group, which have said they are ready to table an amendment enabling Parliament to force ministers to seek a delay if there is no deal in place.
There was speculation at Westminster some ministers opposed to a no-deal break could be prepared to resign to support the amendment when it comes to a vote in two weeks' time.
Meanwhile, a leading business group warned that firms are being "hung out to dry" because of the uncertainty over a no-deal Brexit.
The backbench move comes amid fury among Brexiteers after it was reported the Prime Minister's chief EU negotiator had suggested MPs could be faced with a delay to Brexit unless they back her deal.
ITV News reported that Olly Robbins was overheard in a Brussels hotel bar telling colleagues the EU would probably give the Government an extension to the Article 50 withdrawal process.
He was said to have indicated that if MPs did not vote for a deal, then the delay to the UK's final departure would be "a long one".
"The issue is whether Brussels is clear on the terms of extension. In the end they will probably just give us an extension," he was quoted as saying.
"Got to make them believe that the week beginning end of March... Extension is possible but if they don't vote for the deal then the extension is a long one..."
Conservative party vice-chairman Chris Philp later dismissed the report, telling BBC's Newsnight: "What a civil servant might speculate in a bar after a few drinks is frankly not that important."
However the comments by Mr Robbins will reinforce suspicions among MPs that Mrs May is trying to "run down the clock" in an attempt to force them to back her agreement.
Tory Brexiteer Andrea Jenkyns tweeted: "If true, the PM should stop ignoring the wishes of the British people and disregarding her own red lines."
Former Ukip leader Nigel Farage said Mr Robbins was part of the "Civil Service fifth column" and called for him to be sacked for his combination of "treachery and incompetence".
However Steve Baker, the deputy chairman of the pro-Brexit Tory European Research Group, said it was the view of the Prime Minister which counted.
"Officials advise. Ministers decide. If the PM decides we are leaving on March 29, deal or no deal, that will happen," he said.
A Government spokesman said: "We would not comment on alleged remarks from a private conversation which is said to have been overheard in a hotel bar."
Asked on Wednesday morning whether the reported comments of Mr Robbins reflected Government policy, Brexit Secretary Stephen Barclay said: "No."
Mr Barclay told BBC Radio 4's Today: "The Prime Minister has been very clear that we are committed to leaving on March 29."
The Brexit Secretary, who met senior MEPs in Strasbourg on Tuesday, added: "What came over was actually that it is not in anyone's interests to have an extension without any clarity. It is actually very disruptive to the European Parliament.
"They have obviously elections for the Parliament and a Commission that will be formed at the end of May, so there is no desire on the European side to see what one described to me as an 'extension in darkness', where there is no clarity as to why we are extending."
The row broke as the British Chambers of Commerce (BCC) published a list of 20 "critical" questions it said remained unanswered for business if the UK leaves the EU on March 29 without a deal.
Adam Marshall, director general of the BCC, said: "In less than 50 days, UK firms could face the biggest change to their terms of trade in over a generation, without the information and clarity they need to navigate their forward course.
"There is a very real risk that a lack of clear, actionable information from Government will leave firms, their people and their communities hung out to dry."
Meanwhile it was reported that the Chief of MI6, Alex Younger, is expected to stay on beyond his retirement date to guide the secret intelligence service through Brexit.
Downing Street declined to be drawn on a report that officials want Mr Younger, 55 - who is due to retire in November, to carry on for 12 months to two years after Britain has left the EU.
Meanwhile, the group led by Ms Cooper and Sir Oliver said they were preparing plans to enable Parliament to prevent the country falling into a no-deal Brexit "by accident".
The move follows the announcement by Mrs May on Tuesday that she will make a further Commons statement on February 26 and table an amendable motion to be debated and voted on by MPs the following day, if she has not got a deal at that point.
The group said they would then put down an amendment creating time for a bill requiring the Prime Minister and Parliament to decide by mid-March whether the UK is leaving with a deal, without a deal or whether it will seek an extension to Article 50.
Ms Cooper said the bill creates a "Parliamentary safeguard to prevent us drifting into no-deal by accident".
A similar amendment by Ms Cooper and the Tory former minister Nick Boles was defeated in the Commons last month.
However, supporters of the plan believe that as the calendar counts down towards March 29, it will focus the minds of MPs on all sides who believe no-deal would be a disaster for the UK economy.
Sir Oliver said: "Any MP who genuinely wants to prevent an under-prepared no-deal Brexit will need to vote for this bill at the end of February."
Other MPs supporting the move include the senior Labour MP Hilary Benn, Tory former attorney general Dominic Grieve and the Liberal Democrat Norman Lamb.
It is also backed by Tory MP Dame Caroline Spelman and Labour's Jack Dromey, who last month tabled a non-binding amendment opposing a no-Brexit which did win Commons support.