Mr William Hague told the CBI conference that monetary union could leave British business "trapped in a burning building with no exits". Despite repeated evidence that CBI members favour joining the single currency, Mr Hague travelled to Birmingham to voice "his deep concerns" in the spirit of "candid" friendship.

But the strains on that friendship were all too clear, as Mr Hague likened those rushing to join as "lemmings", telling conference there was more to being "a pro-business party" than simply telling everybody what they wanted to hear.

"A single currency should be low on Europe's list of priorities," he declared. Economic and monetary union was a political idea but, unlike the Exchange Rate Mechanism, was for "all time".

He continued: "British business could find itself trapped in a burning building with no exits. Business needs a hard-headed assessment of the risks involved in a single currency before we can consider joining it, and that assessment is only just beginning."

To those who argued that the currency would happen and Britain could not be left behind, Mr Hague charged: "It is the argument used by lemmings throughout the centuries and it does not bear scrutiny."

The growing sense of Mr Hague's isolation was slightly relieved yesterday by a letter to the London Times by 13 leading business figures voicing their reservations and insisting the case for membership of the currency had not been made.

However, a Gallup survey for the Daily Telegraph said that, while fearful of the possible consequences, most British bosses (over 60 per cent) favoured membership.