Boris Johnson has claimed t he price of Britain quitting the European Union is "lower than it's ever been" despite bleak warnings about the potential impact of a vote to sever ties with Brussels.
The mayor of London insisted that "in an ideal world" he would want to stay in a reformed EU, but he played down the risks of a so-called Brexit.
The senior Tory repeatedly refused to rule out spearheading the campaign for the UK to leave the EU but insisted he had "great faith" in David Cameron's ability to negotiate better terms of membership.
Mr Johnson's intervention, during a visit to Osaka in Japan, came as a campaign to keep the UK in the EU used a high-profile launch event to deliver a stark warning about the potential damage to the UK from a decision to leave.
The mayor told the BBC: "I think I am exactly where the Prime Minister is and, I think, actually a huge number of the proportion of the British public.
"We want, in an ideal world, to stay in a reformed European Union but I think the price of getting out is lower than it's ever been. It's better for us to stay in, but to stay in a reformed EU. That's where I am."
Mr Johnson has so far been careful to support Mr Cameron's position of waiting to see the outcome of the current renegotiation but many Eurosceptics believe his private views are more hardline than the Prime Minister's.
Ukip leader Nigel Farage said he believed the mayor could be persuaded to lead the campaign for Britain to leave the EU.
Asked if that was a possibility, Mr Johnson told the Press Association: "You are now mounting hypothesis upon hypothesis."
Mr Johnson's assessment of the costs of quitting the EU were in contrast to the picture painted by the Britain Stronger in Europe campaign.
At the launch event in east London, former police chief Sir Hugh Orde warned that Britain could be viewed as a safe haven by international criminals if the country voted to end its relationship with Brussels.
Fo rmer Association of Chief Police Officers president Sir Hugh predicted "villains" would come to the UK to escape justice if the country voted to leave, because it would mean pulling out of EU-wide agreements on extradition and sharing evidence.
Speaking at the launch event in east London, he said: "If I was a villain somewhere else in Europe and I was escaping justice, I would be coming here because it's going to take a lot longer to get you back."
Cross-border extradition arrangements would also have to be redrawn, potentially causing huge delays in the justice process, he said.
Sir Hugh added: "Citizens in this country would be put at risk as a consequence. That's not scaremongering, it's simply the fact."
The campaign's figurehead, f ormer Marks and Spencer boss Lord Rose warned of the economic impact of a vote to leave the EU in the referendum promised by Mr Cameron by the end of 2017.
He said: "Those who want us to leave Europe would risk our prosperity, they would, I believe, threaten our safety and they would, I believe, diminish our influence in the world.
"We know our economy would take a hit, what we don't know is how bad that hit would be."
He continued: "They cannot guarantee that jobs would be safe or that prices wouldn't rise.
"They cannot explain how we would stop free movement and simultaneously keep access to the world's largest duty free market.
"They cannot say how our diminished status would impact on our relationships with the US or with China or, indeed, with Commonwealth countries.
"Leaving Europe is a leap into the dark and I don't believe that is a risk that is worth us taking."
He cited Confederation of British Industry (CBI) estimates that the benefits of trade, investment, jobs and lower prices mean membership is worth £3,000 per year on average to every UK household.
Based on an annual contribution to Brussels of around £340 per household, there was a "10-to-one return" for Britons, he claimed.
Lord Rose did not deliver a pre-briefed section of his speech describing those wanting to leave the EU as "quitters" but the campaign insisted that was still their view.
Britain Stronger In Europe executive director Will Straw said: "Stuart Rose edited the speech this morning and read it in his own words. But we have been quite clear that people who leave the EU are quitters."
He added that the In campaign would like the Prime Minister to play a leading role in putting the case for a vote to remain part of the EU.
"Of course we would love the Prime Minister and the Government to campaign to stay in the European Union because we hold a conviction that Britain is stronger, safer and better off in the EU."
Mr Farage rejected the economic argument made by Lord Rose, and accused him of "deliberately" confusing Europe with the EU.
"He argues that in order to trade we have to have free movement of people. Nowhere else in the world is that the case. To think that UK manufactured cars would face tariffs when we import hundreds of thousands more cars from Germany shows utter detachment from the realities of trade."
Dominic Cummings, director of the Vote Leave campaign, referred to the In campaign by its initials BSE and claimed it was " led by people who told us Britain would be doomed unless we ditched the pound".
"The BSE campaign has nothing to offer but the same cliches the Foreign Office has spouted since the Suez disaster," he said. "We need a new UK-EU relationship based on free trade and global networked cooperation so we can adapt fast to the forces that are undermining the EU's 1950s structure."
Mr Cameron will have lunch in Brussels on Thursday with European Commission president Jean-Claude Juncker to discuss progress in the technical talks which have been taking place since June on the UK's demands for reform.
The PM's spokeswoman said the meeting, at the start of a regular summit of the European Council, would be "an opportunity to take stock" of the talks, which have been taking place between the UK, the Commission and the Council Secretariat in Brussels.