Former South Shields MP David Miliband has joined a cross-party bid to prevent a hard Brexit.
Mr Miliband, who quit as an MP after losing the Labour leadership to his brother Ed in 2010, will share a platform tomorrow with Tory ex-education secretary Nicky Morgan and Liberal Democrat former deputy PM Sir Nick Clegg to push for a softer withdrawal from the EU.
The trio said Britain was being "held to ransom" by hardline Brexiteers in a joint article for the Mail on Sunday.
They say: "Less than six months before the deadline for concluding the terms of our departure, hard- Brexit demands are holding the country's negotiating position to ransom.
"A hard Brexit won't create global Britain. It is merely a path to a fantasy island of our own where we will have reduced access to our largest markets and a diminished standing in the world."
The trio insisted they were concerned about the economic impact of withdrawal, stating: "Even a comprehensive free trade agreement with the EU predicted the UK will see growth drop by 5% over the next 15 years compared with current forecasts, as a result of additional bureaucratic border checks.
"The ensuing economic pain will be felt in every part of the country. This is not Project Fear; it is project reality based on the Government's own analysis."
Calling on Parliament to change Brexit legislation, they wrote: "Over the coming months, MPs will have the chance to table amendments to Bills - and vote for those amendments - which can prevent the country from suffering the long-term damage that a hard Brexit will cause.
"Parliamentarians who champion the hardest of Brexits will stubbornly resist any such amendments, and the pro-Brexit press will angrily protest, but what is the role of MPs if not to improve and protect the lives of their constituents?"