The former British chancellor Philip Hammond has said a no-deal Brexit "would be a betrayal" of the 2016 referendum result.
Theresa May's former minister has called on Boris Johnson's government "to demonstrate its commitment to a genuine negotiation with the EU".
Mr Hammond, who resigned from government before Mr Johnson took office last month, has been responding to the new British government's insistence that the UK will leave the EU on October 31st with or without a deal.
Mr Johnson has repeatedly claimed he wants a deal - but has also said a deal can only happen with the removal of the Irish backstop from the withdrawal agreement.
That demand has been firmly rejected by both the Irish Government and senior EU officials.
Writing in The Times, Mr Hammond said the backstop demand represents a move "from a tough negotiating stance to a wrecking one".
He argued: "The unelected people who pull the strings of this government know that this is a demand the EU cannot, and will not, accede to.
"So those of us who desperately want to believe this Conservative prime minister is committed to negotiating a deal that will protect our future prosperity need to see evidence that it is happening soon."
The Conservative MP said no-deal is "not an acceptable outcome", adding that it's important to 'bust' the myth that no-deal is what people voted for in 2016.
He also pointed to warnings from the Irish-American caucus in the US that Congress will not allow a trade deal with the UK if a hard Brexit puts the open Irish border at risk.
He further suggested that no-deal would risk the 'fragile peace' in Northern Ireland, as well as potentially leading to a border poll in Ireland and a second referendum in Scotland.
Mr Hammond's comments come as House of Commons speaker John Bercow warned he would refuse to allow Boris Johnson push through a no-deal exit by bypassing or suspending parliament.
According to The Telegraph, Mr Bercow pledged to fight any such attempt "with every bone in my body".
He's quoted as saying: “We cannot have a situation in which Parliament is shut down - we are a democratic society.
"And Parliament will be heard and nobody is going to get away as far as I am concerned with stopping that happening.”
Main image: File photo of Philip Hammond and Boris Johnson. Picture by: Dominic Lipinski/PA Archive/PA Images