Tony Blair has called for campaigners for Britain to stay in the European Union to make the case with "passion, vigour and determination".
The former prime minister said the UK's destiny is to "lead in Europe" and warned Brexit would lead to economic instability and "damage fundamentally" the interests of the British people.
Mr Blair told the BBC Radio 4 Today Programme: "The question is 'what is in the interests of the people?'
"And 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."
His intervention comes as senior Cabinet minister Iain Duncan Smith accused the Remain camp of making "almost biblical" threats about the consequences of Brexit.
While Labour leader Jeremy Corbyn has faced heavy criticism for being conspicuously absent from the campaign trail, Mr Blair admitted he was "concerned" by the fervour and enthusiasm displayed by the Brexit campaign, and said more passion should be shown by pro-Europeans.
He warned voting to leave would lead to economic uncertainty, and said that, in the end, this argument will win out.
He said: "In the end this referendum will be won for those that see Britain remaining part of the European Union".
But while Mr Blair is an ardent pro-European, he signalled he would not play a central role in the referendum campaign - indicating he knows he remains a hugely divisive figure who might harm the campaign's chances.
He said: "I don't know whether it is the right time for me on the campaign trail, that carries with it negatives as well as positives.
"But I think it is certainly time for people to argue this case for Europe with some passion."
Displaying his frustration with discussions about Brussels, the former prime minister said Britain should be at the very heart of Europe.
He said: "Our destiny as a country is to lead in Europe, and we can and we should.
"And once this is out of the way, with some strong leadership in our own country we will."
But Mr Duncan Smith, the Eurosceptic work and pensions secretary, hit out at claims that Brexit would lead to economic uncertainty and could be damaging.
He said the real uncertainty lay in staying in an EU which is being overwhelmed by the migrant crisis and facing severe economic problems.
He told the BBC Radio 4 Today Programme: "I think the counterproductive thing is frankly the kind of assertions that are being made by those who want to remain in.
"They are almost panicky really, I listen to these endless comments and speeches about the dire warnings, they are almost biblical, you are expecting a plague of frogs and the death of the first born."
The real economic threats are the euro which is in "chaos and collapse" and Greece which is "ready to come back with its begging bowl", the minister warned.
He added: "If there is going to be a period of concern, I have a massive concern.
"If the European Union is so vital to jobs and suddenly we are going to lose jobs, if that's the case, why in the European Union in France, Spain, Italy, Greece, do they have some of the worst youth employment in the world?"
He added: "If it was about simple access to the single market...then every single country in Europe would have incredible employment, they would have no youth unemployment.
"The reality is we are tied to an organisation that is pretty sclerotic."
Instead Britain should look beyond Europe's borders to Asia and the Americas to boost trade, Mr Duncan Smith said.