The result from Crewe & Nantwich is an electric shock of exceptional voltage for the Government. To lose a by-election to the Conservatives for the first time in three decades would have been bad enough in any circumstances. To do so by this margin and on a high turnout (the usual “our people remained at home” line will not do on this occasion), and after a campaign that was more worthy of a student union jape than a serious electoral contest, is far, far worse.

As noted in this space on Thursday, it was an insult to the intelligence of the voters of Cheshire for Labour to select a candidate on the basis of her family ties and then to assail the (considerably more local) Tory contender on the basis of his family heritage. If nothing else (and there should be an awful lot more) that Labour learns from this debacle, it is that the place for top hats and tails is in a store devoted to wedding hire, not an election held in modern Britain.

In a sense, however, absolutely no one should be surprised at this outcome. It is wholly consistent with the national opinion polls, all of which have moved sharply against Labour in the past two months, and it closely matched what had happened not only in this constituency but everywhere polling stations were available in the local and London mayoral elections three weeks ago. If the “toff” charge would not superglue on Boris Johnson, it could not stick to a man whose educational background appears positively commonplace by comparison. This Government was in deep trouble on Wednesday before one ballot paper was cast in Crewe & Nantwich. It finds itself in even deeper trouble this morning.

By-elections have a larger impact than opinion surveys or council contests on the minds of Members of Parliament. Translating abstract numbers into the survival of any individual MP is an effort that demands a pocket calculator. Comparing one's seat to Crewe & Nantwich is a much easier and, in this instance, far more frightening exercise. It is inevitable that there will be immense speculation about the Prime Minister's future within the Labour Party as well as beyond it. It is ludicrous for ministers or Downing Street aides to claim otherwise. It is equally ill-advised to conclude that the solution is for Mr Brown to carry on just as he has been doing since last autumn.

The brutal truth is that the name of the game for this Prime Minister today is survival. It is not a pretty spectacle but it is realistic. Mr Brown has to do well enough in the next 12 months to be certain that his colleagues will retain him in place until and through the next general election. He should be secure, if simply because the queue to sup at what is perceived to be a poisoned chalice is rarely a long one. Mr Brown, nevertheless, has to show the capacity to fare far better. Labour can find its “men in grey suits” if he does not.

The Prime Minister cannot afford any more self-inflicted disasters. The single largest obstacle between the Government and the comparative luxury of the summer parliamentary recess, when it will have the time to regroup and think again, is the forthcoming vote in the Commons on whether to extend the maximum period that terrorist suspects can be in prison without charge from 28 days to 42 days. There cannot be anyone in the Whips' Office who believes that they have the faintest chance of preventing enough Labour MPs from defecting and dooming this measure. Mr Brown, like an officer walking out into the Battle of the Somme, insists that he will proceed.

He needs, instead, to cut his losses. It would be better to withdraw this proposal instantly than to be humbled about it by the House of Commons it and humiliated in the Lords. The case for this change has never been made convincingly.

Mr Brown should, furthermore, be honest. He should admit that he lacks the votes to pass this provision and acknowledge that is why he is abandoning it. He should not seek to dream up a limp excuse relating to alleged new information about terrorism or imply darkly that David Cameron is in league with Osama bin Laden to imperil the security of British citizens. He should concede and move on and thus at least buy himself some time. This is not the most heroic of political strategies. But sometimes mere survival is an end in itself.