Blair, who served as Labour prime minister from 1997 to 2007, said he was concerned about the result of the referendum and cautioned Britain would face years of uncertainty if it left the world's biggest trading bloc.
"You can't have a referendum like this, especially with the fervour on the anti side, and not be concerned, so I am
concerned, but I believe in the end we will vote to stay," Blair told BBC radio.
Prime Minister David Cameron is leading the push to stay within the bloc, but polls show Britain is divided about membership with a large number of undecided voters.
"I would like to see the pro-European side get out there with a bit of passion and vigour and determination and stand up for what we believe - and what we believe not just as a matter of economic realism, but as a matter of political idealism," he said.
Blair said that swings in the value of sterling over the prospect of an exit had already shown the type of uncertainty that would arise if there was a vote to leave the bloc.
"As you can see from the gyrations on the currency markets, the one thing that is for sure is that if you vote to go there will be several years of uncertainty," Blair said.
He said that the uncertainty would be compounded by questions about the integrity of the United Kingdom.
The Scottish National Party has said it will push for a fresh referendum on independence if Britain votes to leave the EU.
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