Kenneth Clarke, the veteran pro-European former Tory chancellor, has warned that Britain would make a "fatal mistake" if it voted to leave the EU in the referendum promised by David Cameron.
In a direct challenge to the prime minister, who is pledging to hold the poll after repatriating a raft of powers from the EU by the end of 2017, Clarke said Britain should focus on the positive benefits of the EU.
"We need to concentrate on what we are in favour of and just what we are against," Clarke said at the launch of a new cross-party group, British Influence.
The former chancellor re-enacted one of the most controversial episodes of his career on Wednesday evening when he shared a platform with Lord Mandelson and the Treasury chief secretary Danny Alexander at the launch of the new group.
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 was 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 that 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."
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