Former foreign secretary David Miliband has urged MPs of all parties to come together to prevent Britain leaving the EU in a "hard Brexit".
The ex-South Shields MP and Labour cabinet minister said the country was being "held to ransom" by the demands of the hard Brexiteers threatening jobs and living standards in the UK.
In a rare intervention in UK politics, Mr Miliband, who now heads the New York-based International Rescue Committee aid agency, is due to appear on a platform with Liberal Democrat former deputy minister Sir Nick Clegg and the Conservative chairwoman of the Commons Treasury Committee, Nicky Morgan.
However, in an interview with the BBC Radio 4 Today programme, he denied he is seeking to launch a new, centrist political movement.
"This is about calling for MPs of all parties to stand together," he said.
"The truth is that Britain is being held to ransom by the demand for hard Brexit, for severing the links with the customs union and the single market that is at the heart of the debate going on at the moment.
"It think that it is significant that people should come together, frankly out of alarm at the prospects for the UK.
"The prospects of a hard Brexit are very high now and, what's worse, the prospect of no deal is rising too. It does not need to be like this.
"It is very, very important that the country is not held to the kind of ransom that at the moment threatens living standards as well as the political influence of the UK in a very fundamental way."
Mr Miliband said currently the Government has no settled position on what kind of future relationship it wants with the EU.
"The negotiation is not with Europe at the moment; the negotiation is in the Cabinet room. This isn't funny. We are less than 120 days away from the final negotiation," he said.
"There are matters of trade, there are also, frankly, very fundamental matters of conflict on the island of Ireland. These need to be addressed and they should be addressed in a cross-party way."
Mr Miliband was also sharply critical of Jeremy Corbyn over Labour's refusal to support a Lords amendment for Britain to become part of the European Economic Area after it leaves the EU like countries such as Norway.
"The warning for Jeremy Corbyn is that, if he is not very careful, he will be the midwife of a hard Brexit that threatens the living standards of the very people that he says he wants to stand up for and represent," he said.