The former Labour leader has largely kept out of the debate, amid claims he could be seen as a liability for the No campaign.
But at an event yesterday marking 20 years since he became leader of the party, he was asked if he believed Scots would back the Union. “I hope so and I believe so,” he said.
“I think the arguments of the Better Together campaign have got stronger as time has gone on … [among] most people I have talked to there’s some optimism.”
The low profile maintained throughout the independence debate by Mr Blair, who was educated at Fettes College in Edinburgh, is in contrast with the stance taken by his former chancellor Gordon Brown, who has made a series of interventions in recent weeks.
The Nationalists immediately hailed Mr Blair’s intervention as a boost for the Yes campaign.
The SNP’s Westminster leader, Angus Robertson, said: “Mr Blair has been kept well away from the referendum debate by the No campaign for obvious reasons – until now. His illegal war in Iraq was a calamity, and an avoidable one at that and the people of Scotland will not forget it.
“A Labour government – with the full support of the Tory opposition – taking the country to war on the basis of false pretences, with enormous loss of life, displayed the absolute worst side of the Westminster system.
“Tony Blair speaking up for No underlines that only with a Yes vote in September can Scotland avoid any future illegal Westminster wars.”