Tony Blair has urged pro-EU campaigners to get out and make the case for Britain to remain with "passion, vigour and determination".
The former prime minister said he was "concerned" at the fervour and enthusiasm being shown by the Brexit campaign and called on the In camp to demonstrate their "political idealism".
His intervention came as Boris Johnson appealed to voters to ignore the "pessimists" and "gloomadon-poppers" and opt for a future outside the EU in the referendum on June 23.
Amid continued criticism that the Remain campaign has been unremittingly negative, focusing on the economic perils of withdrawal, Mr Blair said they needed to start making the positive case for EU membership.
"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," he told the BBC Radio 4 Today programme.
"And what we believe not just as a matter of economic realism, but as a matter of political idealism."
Mr Blair signalled, however, that he would not play a central role in the campaign - acknowledging that he remains a highly divisive figure who might harm Remain's chances.
"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," he said.
Appearing at a Vote Leave campaign event for the first time, Mr Johnson said withdrawal from the EU could usher in a new era of prosperity for the UK.
"I think it is time to ignore the pessimists and the merchants of gloom and to do a new deal that would be good for Britain and good for Europe too," he said in a speech to workers at a transport depot in Dartford, Kent.
"It is time to burst loose of all those regulations and get out into a world that is changing and growing and becoming more exciting the whole time.
"If we hold our nerve and we are not timid and we are not cowed by the gloomadon-poppers on the Remain campaign and we vote for freedom and for the restoration of democracy, then I believe that this country will continue to grow and prosper and thrive as never before."
His comments appeared to be a direct rebuff to Prime Minister David Cameron, who warned on Thursday of potential large-scale job losses if Britain left the EU and accused the Out campaign of treating it as a "price worth paying".
The strains within the Cabinet were again highlighted as the pro-Brexit Work and Pensions Secretary Iain Duncan Smith poured scorn on the warnings of the In campaign.
"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," he told the Today programme.
Downing Street however insisted that Mr Cameron was right to underline the economic risks involved in leaving.
"He thinks that it is important that the Government puts out the facts and the case to the British people," the Prime Minister's official spokeswoman said.
"When the Government has been so focused on making sure that we deliver economic and financial security for people up and down the country, it is important that we highlight the risks of leaving."