It can come as no surprise to ministers that the astonishing loss of confidential data by Her Majesty’s Revenue & Customs revealed on Tuesday has been a hammer blow to their reputation. If anything, that a mere 64 per cent of those surveyed said that the incident had called into question the “basic competence” of the Government was a charitable response, especially as it becomes plain that what occurred was far from an isolated incident involving an inept individual but part of a cultural weakness on information that deserved the strongest protection.

Even so, the extent to which this event has deflated confidence in Gordon Brown and his administration across the board is striking. The proportion of the public that believes this comparatively new team is competent and capable, has a good set of leaders or is honest and principled has essentially been halved in two months. The Prime Minister’s standing with the country has moved from a honeymoon to trial separation without cohabitation. This is an extraordinary transformation and one that raises the question of whether Mr Brown recognises the national mood, never mind whether he can adapt to it.

The most deluded members of the Downing Street inner circle might divine a degree of comfort from aspects of these findings. The public is reluctant to blame Alistair Darling personally for what has happened. The Conservatives are clearly gaining in some categories, but not all. There is precedent for voters shifting their views radically because of an adverse development — the fuel crisis of September 2000, for instance — yet returning to their old attitudes shortly afterwards.

Mr Brown should not listen to any such reassurance. It is profoundly mistaken. His Chancellor could easily be fatally damaged if more examples of unheeded warnings on data emerge. As a rule, oppositions do not win elections, it is governments of the day which lose them: so it is almost irrelevant whether or not the electorate thinks the Conservatives would be much more competent at the moment. The fuel crisis might have prompted short-term anger with Tony Blair but it did not alter broader overall perceptions of the likely future direction of the British economy.

For the real live wire in this survey lies less in what voters deem of management procedures at Revenue & Customs (which will, in time, probably pass) but the extent to which the Northern Rock saga and the wider sense that the economy as a whole is at risk in the coming year has undermined economic optimism. The numbers here have shifted from being mildly positive (53-45 per cent upbeat) to decisively negative (55-34 per cent downbeat) in eight weeks.

The return of the Northern Rock drama has again raised doubts about the Government’s long-term economic management and its short-term competence. From a credit crunch has emerged a credibility crunch. The experts may be predicting a slowdown in economic growth rather than something truly dire, but if the public is wary of harder times, then the retrenchment of expenditure will itself harm the economy and with it the Government’s prospects.

Mr Brown has built his career on a record as Chancellor, which, while not as stellar as he chose to claim, was always robust enough for voters to prefer Labour. If that position is not regained in the months ahead, he will have nothing to fall back on. This poll points the way to a funda- mental change in Britain’s political preference.