Tony Blair has urged Labour to team up with Tory rebels and force the Government to stay in a customs union with the EU.
The former prime minister used his latest intervention over Brexit to warn of "economic dislocation", express fears of a return to violence in Northern Ireland, and repeat his call for a second EU referendum.
Speaking to Sky News, Mr Blair claimed there are "at least four different types of Brexit now being debated in Parliament" as he rubbished Theresa May's oft-used phrase "Brexit means Brexit".
This week, Tory Remainer rebels - led by ex-minister Anna Soubry - have raised the prospect of a Government defeat over the Prime Minister's plan not to join any form of customs union with the EU after Brexit.
In a tabled amendment to the Trade Bill, Ms Soubry is hoping to alter the legislation in order to compel the Government to retain a customs union with the EU after leaving the bloc.
She has cited cross-party support for her move, with Mr Blair calling on the Labour Party as a whole to join with Ms Soubry's group of Tory rebels.
"It shouldn't be a matter of Labour or Conservative, it should be a matter of what people really think is in the interests of the country," he said.
"If there is an amendment that is going to unite some people in the Conservative Party with the Labour Party, yes we should be all for it."
The Labour Party itself is expected to formally back remaining in a customs union with the EU, with Jeremy Corbyn due to deliver a major Brexit speech on Monday after coming under pressure to clarify his party's position.
Mr Blair welcomed the "good evolution of policy" but admitted he wants Labour to "go further".
He also stated how Labour would be accepting the UK would not be signing new trade deals with non-EU countries after Brexit.
"If you're in a comprehensive customs union, between the UK and the EU, you're not really going to be striking free trade deals with other countries," he said.
"And, in so far as you are in any form of customs union, you are going make yourself much less attractive to those other countries to strike a trade deal."
The ex-premier cited fears of political instability on the island of Ireland due to Mrs May's decision to rule out any form of customs union with the EU, which he claimed would see the establishment of a "hard border" between Northern Ireland and the Republic of Ireland.
"This, I think everyone accepts, the Government accepts, would be a disaster for relations between the two countries and for the Good Friday Agreement," Mr Blair said.
"So what's the answer? If you're not in some form of customs union with the EU, you're going to be with a hard border and you're going to disrupt significantly the trade between the Republic of Ireland and Northern Ireland."
Mr Blair recalled how he would "wake up every morning to the news from Northern Ireland of terrorist acts, of deaths, of destruction" prior to the signing of the Good Friday Agreement in 1998.
"You can't put that at risk," he said.
"A major part of the agreement was precisely on the basis that the Republic of Ireland and Northern Ireland would have an open border.
"So that there would be an acceptance that even though Northern Ireland was staying inside the UK, nonetheless there was a recognition of the politics of the island of Ireland.
"If you break that up you're doing something really irresponsible."
On Thursday, the Prime Minister hauled senior members of her Cabinet to her Chequers country retreat to thrash out an agreed position for the next stage of Brexit negotiations.
Mr Blair claimed divisions within both Cabinet and Parliament on the possible future UK-EU relationship showed how "there are different versions of Brexit", as he repeated his demand for a second vote on Britain's EU membership.
He suggested it is "increasingly" possible there won't be a majority in Parliament "for any particular type of Brexit" due to the "impasse" among MPs.
"At that point, the sensible thing - since this whole Brexit business originated from the referendum in June 2016 - is to say it's got to go back to the people to decide," he added.
"Do they want what is on offer from the Government now as opposed to what we have now in the EU?"
Mr Blair spoke to Sky News as he released a new article titled "Brexit: The Realities of 'Taking Back Control'".
He said his paper shows how there is a global "consensus" that a "clean break Brexit" will result in "significant economic damage" to the UK.