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The significance of Nigel Lawson's intervention is not just that he has broken something of a Tory taboo by calling for Britain to quit; it's also that he is a former chancellor arguing essentially on economic grounds.

The EU, he claims, is hurting one of our most important industries - financial services - and, secure "within the warm embrace of the European single market", giving British businesses an excuse not to develop trade with the developing economies.

Awkward you might think for the prime minister, but today he was putting on a brave face, claiming that Lord Lawson had in fact helped highlight his pledge of an EU referendum if he is re-elected.

His backbenchers want him to promise a Commons vote on the issue before the next election, but there will be no such promise in the Queen's Speech tomorrow.

Like Labour's Harold Wilson, the man in charge the last time Britain had a referendum, David Cameron has promised to renegotiate Britain's relationship with Europe.

Lord Lawson said that would be as pointless in future as it had been then.

What was once unsayable by any senior Conservative has now been said. Which means the issue once described by the foreign secretary as a ticking bomb is ticking rather louder.

Mr Cameron once warned his party to stop obsessing about Europe. The call by a former Tory heavyweight for Britain to leave the EU has made that a forlorn hope.