It is inconceivable that David Cameron would make the case for leaving the European Union, the veteran pro-European former Tory chancellor Kenneth Clarke has said.
Following the launch of a cross-party pro-EU campaign on Wednesday night, Clarke said he believed Cameron had committed himself to the European cause and would be unable to argue otherwise.
"I don't think he [David Cameron] could conceivably start urging people to vote no after all he said about his reasons for being members of the European Union," he said.
Speaking on Radio 4's Today programme, Clarke added that the number of Tory MPs who wanted to quit EU was tiny.
"There's a broad range of opinion inside the [Conservative] party. The number of people who actually want to leave the European Union; it's quite tiny. They get a disproportionate amount of attention. My guess is that there are about 30 who want to leave and when we first joined the European Community I think there was slightly more than that."
He warned that it would be "pretty catastrophic" if Britain left the EU and said he was now resigned to fighting a referendum on the issue if the Conservatives win the next election.
"The background climate in this county has become … unremittingly hostile. I think somebody has got to make the positive case again. The climate of public opinion just needs to be reminded how essential it is if we really want the UK to play a part in the modern world," he said.
At Wednesday night's campaign Clarke re-enacted one of the most controversial episodes of his career when he shared a platform with Lord Mandelson and the Treasury chief secretary, Danny Alexander, at the launch of the new group, British Influence.
Alexander was press officer for the Britain in Europe campaign group in 1999 when Clarke killed off his chances of ever leading the Tory party by sharing a platform with Tony Blair and Gordon Brown.
In his speech Clarke said that Britain would make a "fatal mistake" if it voted to leave the EU in the referendum promised by David Cameron.
But he was also careful to praise the prime minister, who has recently asked him to help press for a new EU-US free trade agreement. Clarke held talks on this at a recent EU-Latin America summit in Chile with Karel De Gucht, the European trade commissioner.
The former chancellor said Cameron was right in his landmark speech last week on Britain and Europe to say that the EU could reform. But he made it clear that Britain must be an active player.
"When the UK plays an active leading role in Europe we can achieve enormous things which we simply could not on our own," Clarke said. "So the real challenge for us as we debate our role in the EU is working out what we should be pushing forward, what we should be leading on within the EU, to make it – and us – a more effective economic and global power."
Clarke called on pro-Europeans to make the case for the EU more effectively as he called for the fulfilment of Margaret Thatcher's two key European ambitions – completing the single market and expanding the EU by admitting Turkey.
"The time has obviously now come for us to put the case more strongly and more coherently," he said. "It is in our vital national interest that we avoid the fatal mistake that would be a no vote if a referendum is held in the next few years."
In his speech Mandelson claimed Eurosceptics had got away with "murder". The former European trade commissioner said: "For far too long, those who want to destroy Britain's interests and influence in Europe have been allowed to get away with murder with the lies and false propaganda they have poured out about the European Union and what it represents for our country. This cannot go unchallenged any more. The pro-Europeans have bided their time. Now we must unbide our time."
Alexander made clear the Liberal Democrats' deep unease at the prime minister's plan when he warned of the dangers posed by those who "put anti-European ideology ahead of our national interest". He said: "We cannot afford to give the impression that we are going to disengage. Britain's national interest demands that we maximise our influence and use that influence to boost growth, trade and jobs. That is the only way to protect British jobs. It's a position that is pro-business and pro-Britain too. It should be obvious to all that we are more powerful negotiating from inside Europe than from the sidelines."