Mr Blair said: “I believe passionately that leaving Europe would leave Britain diminished in the world, do significant damage to our economy and, less obviously but just as important to our future, would go against the very qualities and ambitions that mark us out still as a great global nation.”
He also said an in/out vote would be a “huge distraction” and an “unacceptable gamble” over Britain’s future.
Speaking in his former constituency of Sedgefield, Mr Blair added: “And the oddest thing of all about David Cameron’s position? The PM doesn’t really believe we should leave Europe; not even the Europe as it is today.
“This was a concession to party, a manoeuvre to access some of the UKIP vote, a sop to the rampant anti-Europe feeling of parts of the media. This issue, touching as it does the country’s future, is too important to be traded like this.”
He went on: “It is greatly to Ed Miliband’s credit that he resolutely refused to make that trade.
“He faced down calls to follow the Tory concession from the media and many inside our party.
“In doing so, he showed real leadership.”He showed that he would put the interests of the country first.
“He showed that on this, as on other issues, he is his own man, with his own convictions and determined to follow them even when they go against the tide. I respect that.”
Mr Blair warned: “Think of the chaos produced by the possibility never mind the reality of Britain quitting Europe.
“Jobs that are secure suddenly insecure; investment decisions postponed or cancelled; a pall of unpredictability hanging over the British economy.
“Do we really think this is the time in which to put into play our very membership of the European Union, the largest commercial market and most developed political union in the world? And the one on our doorstep?
“So that instead of playing a leading role in resolving these common challenges, we would decide to engage in the juddering impact of a negotiation which would weaken all these alliances and put us out of the leadership game?
“A decision to quit Europe would say a lot about us and none of it good.”
He said: “Labour and its Leader took a brave decision when they decided not to yield to pressure but instead to make the principled and intelligent case for Britain in Europe.
“In doing so, they showed that they understand Britain’s future and its destiny better than those prepared to trade policy for political advantage. That is one very good reason, amongst many others, for voting Labour on May 7.
“I want Labour to win. I want us to win for the future of our country and its place in the world. I want Labour, under Ed’s leadership, to be the Government of our country on May 8. I believe we can and will do it.”