Five years after stepping down as an MP after losing the Labour leadership to his brother Ed, he will share a platform on Monday with Tory ex-education secretary Nicky Morgan and Lib Dem former deputy PM Sir Nick Clegg.
They will be urging the Government to take a softer approach to withdrawing 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 wrote: “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 a 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.”
Mr Miliband, Tony Blair’s former head of policy, has been living in New York City since stepping down as an MP, working as chief executive of the International Rescue Committee.
He has occasionally dipped his toe in the Westminster political pool since 2013 but this is the former shadow foreign secretary’s biggest intervention yet.
The 52-year-old and the two former leading politicians said their motivation for urging a softer Brexit is based on concern for the economic impact.
The trio said: “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.”
They have called on Parliament to alter Brexit legislation, adding: “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?”
Former Downing Street spin doctor Alastair Campbell, who worked under Tony Blair, said: “I think what you’re seeing right across politics is a real growing concern that the government and parliament are pursuing strategies that are going to fundamentally damage the country.”
Mr Miliband has become increasingly vocal in recent weeks about the Labour leadership and Brexit.
In contrast to the official line, he called on his former Labour colleagues to back calls for a new Brexit referendum, saying the only way to “avert the damage of Brexit” was to give the public another say on a final deal.