The government must commit to a “genuine negotiation with the EU”, former Chancellor Philip Hammond has said.
In his first comments since stepping down last month, Mr Hammond said a no-deal Brexit would “break up the UK”.
“The reality would be a diminished and inward-looking little England,” he said in an article in the Times.
A no deal would be a “betrayal of the 2016 referendum”, he said, adding “it must not happen”.
Prime Minister Boris Johnson has called for the EU to remove the Irish border backstop plan before the Brexit deadline of 31 October.
Many of those who voted against Theresa May’s Brexit deal had concerns over the backstop – designed to guarantee there will not be a hard Irish border after Brexit – which if implemented, would see Northern Ireland staying aligned to some rules of the EU single market.
In his article, Mr Hammond said: “The pivot from demanding changes to the backstop to demanding its total removal is a pivot from a tough negotiating stance to a wrecking one: the unelected people who pull the strings of this government know that this is a demand the EU cannot, and will not, accede to.”
He said it was a “myth that a no-deal exit will be painless” and that “all credible economic analysis shows that the losses will far exceed the potential benefits”.
“There is no popular mandate for a No Deal Brexit; and no Parliamentary mandate for a No Deal Brexit,” he added.
‘Red lines alarming’
Mr Hammond said: “Most people in this country want to see us leave in a smooth and orderly fashion that will not disrupt lives, cost jobs or diminish living standards, whether they voted Leave or Remain in 2016.
“Parliament faithfully reflects the view of that majority and it will make its voice heard.”
Mr Hammond’s comments come as Downing Street said it expects a group of MPs to try to block a no-deal Brexit by attempting to pass legislation when Parliament returns next month.
Speaking on Tuesday, the Commons Speaker John Bercow said he believed MPs could stop a no-deal Brexit and pledged to refuse to let Mr Johnson suspend Parliament to achieve such a course.
Earlier, Work and Pensions Secretary Amber Rudd urged Mr Johnson not to force through a suspension.
She told the BBC: “I remain a great admirer of Parliament and of parliamentary sovereignty and I will continue to argue for the executive of the government that I’m part of to work with Parliament, not against them.”
Meanwhile, the Sun is reporting Mr Hammond and 20 other senior Tory MPs have written to the prime minister to say his demand to scrap the Irish backstop “set the bar so high that there is no realistic probability of a deal being done”.
They said they were “alarmed by the ‘Red Lines’ you have drawn which, on the face of it appear to eliminate the chance of reaching agreement with the EU”.
The group also demands Mr Johnson declares he is firmly committed to leaving the EU with a deal and is ready to compromise to get one – pointing out those were assurances he gave during the leadership campaign “both publicly and privately”.
Seven other former cabinet ministers are said to have signed the letter, including David Lidington, David Gauke, Rory Stewart and Greg Clark, all of who resigned before Mr Johnson took office.