WHO to believe? The Ipsos Mori research company undertakes an annual survey of the opinion of leaders of British industry about economic prospects. The result of the most recent survey, taken about two weeks ago and published yesterday, shows that corporate opinion in Britain is more optimistic about the economy improving than it has been at any time in the past six years.

If the comparison had been between one year ago and now the optimism, even if qualified, would have been understandable. But what the chairmen and chief executives appear to be saying is that prospects are better than they were in, say, 2004 and 2005 before the credit-crunch had been heard of and talk of a global recession was nowhere on the horizon.

This research will certainly bring some relief to Gordon Brown and Alistair Darling whose forecasts of the first sights of green shoots in the final quarter of this dying year have not been fulfilled, but it has to be set beside against other evidence that is less encouraging. More than one prime minister in the past has been caught out by unexpected negative economic or other factors when choosing the date for an election. Mr Brown has to decide whether the chance of avoiding the same fate will be greater in March than in May -- not an easy calculation.