"Once this thing has been started by a referendum it can frankly only be finished by a fresh vote," he said.
Blair left office in 2007 and spent many of the following years abroad, including as an international envoy to the Middle East.
But these days he is more often found in London, where he has plunged back into British politics.
"I'm passionately opposed to Brexit and I still believe it can be changed," the 65-year-old said in the offices of his non-profit organisation, the Institute for Global Change.
After two years of wrangling with her Conservative party, May finally presented her plan this month for economic ties with the EU after Brexit, sparking outrage among hardliners in the party for giving too much away to the EU.
Blair himself said it was a "mush", an "incomplete half-in half-out" plan that pleased no one -- and was unlikely to be accepted by Brussels. He noted the inherent dilemma in Brexit -- stay close to the EU to protect trade but forfeit the opportunities of going it alone, or cut ties altogether and risk damage to the economy.
With parliament "paralysed" on the way forward, "the only way, in the end, this is going to be resolved is putting it back to the people", he said.
Blair's interventions on Brexit have not always been well received in Britain, where his decision to join the United States in invading Iraq in 2003 remains hugely controversial.
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