The former Prime Minister – who confessed both US and UK politics leave him “baffled” – said that Scotland could cede from the UK if we leave the EU.
Mr Blair, 62, said that a vote to quit would “change the debate around Scottish independence”.
Scottish Nationalists have warned that they will push for a second referendum to break up Britain if the Remain camp does not win.
His comments align with the “project fear” narrative that the Remain camp has pushed since the campaign got underway.
Mr Blair told the Guardian: “This isn’t just about the UK and Europe, it’s about the UK, because if the UK voted to leave Europe it would completely change the debate around Scottish independence.
“Therefore you actually are talking about the future of Britain, you’re not just talking about the future of Britain in Europe.”
He also warned that Brexit could have consequences for the British economy and that the Remain case is “overwhelming”.
Mr Blair said: “The case for Britain staying in Europe is overwhelming because leaving the largest commercial market in the world, the biggest single political union in the world right on our doorstep, in the 21st century would be a huge leap of uncertainty and difficulty”.
His comments were undercut by his confession moments before that his political judgement is way out of line with today”s electorate.
He said: “I really mean it when I say I’m not sure I fully understand politics right now, which is an odd thing to say, having spent my life in it.”
Mr Blair”s admission came as he marvelled at the rise of populist left-wingers like US senator Bernie Sanders, and Jeremy Corbyn in his own party.
The former three-term Prime Minister tried to stop Labour electing Mr Corbyn its leader, but failed to swing the vote.