A strong performance by the Prime Minister in the House of Commons, followed by a forceful statement from Charles Clarke, may have been enough to ease the pressure on the embattled Home Secretary. It cannot, though, be satisfactory that at least 38 of those who should have been considered for deportation for the most serious categories of crimes remain at large. It would surely help the police in their quest to find these people if the Home Office released the names, any photographs and last-known addresses of those whom they are now seeking. Mr Clarke has not produced a compelling explanation for this omission.

Part of Tony Blair and Mr Clarke’s political success yesterday involved a strategy of diversion. Both men sought to shift the focus away from the inept administration of the current system for assessing deportation towards the character of that system itself. Each insisted that too few of those who are theoretically eligible for deportation actually depart the country. Mr Blair said that those guilty of imprisonable offences should “automatically” leave. Mr Clarke called for a “clear presumption” that deportation would occur.

There is little reason to believe that the Home Office would have overseen a tough regime any more impressively than it did the present arrangements. There is, nonetheless, a strong case for looking at this area again. The sharp rise in the numbers and the proportion of all prisoners who are foreign nationals would alone, irrespective of this scandal, represent a reason for asking if the deportation rules are lenient. This is, though, complicated territory. A swift dash to legislate, motivated by the quest to salvage a political career or defuse a crisis, would be mistaken.

Foreign national prisoners include a wide variety of sins and of sinners. All imprisonable offences range from persistent failure to pay the television licence fee to multiple murder. It is doubtful whether a judiciary that is increasingly prone to ruling not only on the letter of a law but whether its content is “equitable”, “fair” and “proportionate” would allow such an initiative to proceed without a challenge. It would be ludicrous to frame a law that could not withstand such scrutiny.

The “foreign” element of a foreign national prisoner is also not uniform. There is a world of difference between a Canadian who has been resident in Britain for three decades and brought up a family but has become involved in a pub brawl and, for example, a Russian who has been here for a few months and spent his time selling illegal drugs. A reformed formula for deportation has to have room for discretion, recognise the reality that Britain’s ability to deal with prisoners from other EU states is partly constrained by Union regulations and take note of international treaty obligations that enter the equation.

The Home Secretary is thus right to call for a “clear presumption” of deportation, especially for the worst types of crimes, and the main opposition parties should back him on that principle. There will be, as Mr Clarke concedes, “special circumstances” when deportations do not take place and ministers should not imply to the public that these will be very rare when there is every chance that the figures will not be negligible. The best way of securing legislation that the courts will tolerate is to think through the consequences. A Home Office that has yet to show it can make existing rules work should breathe deeply before rewriting them.