The almost Labour leader and ex-foreign secretary David Miliband has offered reluctant support to the chancellor Philip Hammond over his push for a transition period after Brexit and called for a second vote.
"I never thought I would say this, but the chancellor, Philip Hammond, is also playing a valiant role. The transition he supports is vital," he said, writing in the Observer.
"However, a transition postpones a rupture rather than avoiding it. Slow Brexit does not mean soft Brexit. Steve Baker, minister in the department leading the negotiations, has been refreshingly honest in saying the transition period is a 'soft landing for a hard Brexit'. We have been warned."
Miliband, now president and chief executive of New York-based International Rescue Committee, warned that the terms of a final Brexit deal should be subject to a second vote, either by the public via a second referendum or a vote in Parliament.
He reserved criticism for Hammond's colleagues however, and the Prime Minister.
"People say we must respect the referendum. We should. But democracy did not end on 23 June 2016. The referendum will be no excuse if the country is driven off a cliff," he said.
"MPs are there to exercise judgment. Delegating to Theresa May and David Davis, never mind Boris Johnson and Liam Fox, the settlement of a workable alternative to EU membership is a delusion, not just an abdication.
"Brexit is an unparalleled act of economic self-harm. But it was a big mistake to reduce the referendum to this question. The EU represents a vision of society and politics, not just economics. We need to fight on this ground too."
He added: "Ministers cannot even agree among themselves the destination, the route map or the vehicles to get us there."
It comes as Hammond and trade secretary Liam Fox appeared to put weeks of infighting behind them by agreeing on a "time-limited" transition deal to avoid a Brexit "cliff-edge".