Former chancellor Lord Darling has told a Lords committee the biggest stumbling block in Brexit talks is "that we don't know where we going".
The Remain-supporting peer compared the government's current strategy to "being in a snowstorm" warning we could "stumble off the edge of a cliff" at any moment.
He rubbished the idea that any "planning" was being made because ministers' responses vary from day to day, although noted there was "a limit to what you can do" in any case.
"Maybe there's a cunning plan yet to be revealed, but we don't seem to be getting far with the three things on table," Darling added, in reference to the failure to make sufficient progress on citizens rights, Northern Ireland or the divorce bill.
Darling, who campaigned to Remain alongside his successor George Osborne, said he was sometimes asked by people on the other side of Channel "what do you guys want" and "the answer is we don't really know".
The second most serious threat is that "time is rapidly running out" and that the value of a transitional deal "degrades every day", the Labour peer added.
Darling warned that the EU27 could "sit it out" longer than the UK, and that a transition of 18 months would not be enough. "You'd need two, three, four years," he told the Lords. "I can't see how you'd get 40 years of stuff done in 18 months."
But while he made stark warnings about the impact of a no deal, noting that "even North Korea" has some trade deals in place, Darling was begrudgingly hopeful about a deal being reached. He said it would involve spending more time in Brussels to build "political rapport" and stressed the importance of German Chancellor Angela Merkel in the talks.
"We are almost part of same continent, how can we not get on?" he said.