Tony Blair stepped up his warnings on no-deal Brexit today saying it would be 'devastating' for Northern Ireland and breach the Good Friday Agreement.
The former PM said crashing out of the EU would inevitably lead to a 'really hard border' on the island of Ireland and cause a huge split within the UK.
Despite a series of setbacks for those campaigning for another Brexit referendum, Mr Blair said still hoped one might happen when people saw the 'true alternatives' the country faces.
The latest intervention from Mr Blair comes as Theresa May struggles to get concessions from the EU that can win support from MPs for her deal - with less than seven weeks to go until the UK is due to leave the bloc.
Tony Blair (pictured giving an interview on Sky News) has stepped up his warnings on no-deal Brexit saying it would be 'devastating' for Northern Ireland
Mrs May has so far been met by a wall of resistance from Brussels, which insists the Withdrawal Agreement will not be reopened.
The PM is facing another round of crunch Brexit votes in the Commons on Valentine's Day - with Remainer and Eurosceptic opponents again seeking to bind her hands.
Former Labour leader Mr Blair heaped pressure on Mrs May this morning by issuing another dire warning about the consequences of leaving without a deal.
'No one could responsibly propose (a no-deal Brexit). It would be economically very, very dangerous for Britain, and for the peace process in Ireland it would potentially be devastating, ' he told Sky News' Sophy Ridge On Sunday.
'You would have a hard border, very hard border. A no-deal Brexit means a really hard border between north and south in Ireland, it's contrary to the Good Friday Agreement and it will cause an enormous fissure within the United Kingdom.'
Mr Blair, who has argued strongly for a second EU referendum, added: 'I've never thought that you would get to another referendum going directly to it - you'll get to it when people see what the true Brexit alternatives are. And the truth is there are two.
'You can have a soft Brexit, which is really what Jeremy Corbyn is suggesting, or you can have the hard Brexit that Boris Johnson and Nigel Farage and other people want.'
He said the type of Brexit must be decided before the UK leaves the EU, arguing that a lack of clarity would lead to a lack of 'closure'.
He said: 'The argument just goes on, and by then you'll have left, you'll have paid your money up front, you'll have given up your negotiating leverage.
'And for the country to do that, as Theresa May wants to do - to leave without knowing what Brexit you get - this would be, in my view, an incredibly foolish thing for the country to do. It's got to know where we're heading before we leave.'
Theresa May (pictured with Jean-Claude Juncker in Brussels last week) has been struggling to get concessions from the EU that can win support from MPs for her deal
Theresa May (pictured with husband Philip at church in Maidenhead today) has promised another 'meaningful vote' in the coming weeks, but Labour is determined to reduce her wriggle room