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.
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.
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.