Leading article: Facts, not prejudice, should inform this debate

Together, the words "illegal" and "immigration" are always an incendiary mix. Shortly before the last election, the Prime Minister got into trouble over his refusal to estimate the number of illegal immigrants in Britain, despite being pressed on the BBC's Newsnight by a particularly insistent Jeremy Paxman. Now, a row has blown up following the reply to a similar question given by the head of enforcement and removals at the Immigration and Nationality Directorate. Asked how many people he thought were in the UK illegally, Dave Roberts said he hadn't "the faintest idea".

Cue cries of "scandal", "disgrace" and the rest from all the familiar quarters. It is not the answer that constitutes the scandal and disgrace, however, but the question, which is ridiculous. There cannot be an accurate answer for the simple reason that individuals with no right to be in Britain are unlikely to register the fact. The longer they can keep their heads down, minimise their dealings with the state and keep up an appearance of normality - by working, for instance - the better their chances of remaining undetected. The only honest answer is the one Mr Roberts gave.

But this, of course, is not good enough for those seeking to make political capital from the immigration issue by highlighting illegality. The Nigerian heart patient, cast as a "health tourist" and queue-jumper by sections of the press offers a case in point. Ese Elizabeth Alabi died at Papworth hospital earlier this week from heart failure. She was waiting for the High Court to rule on her plea for a heart transplant. She had been denied priority status because her visa had expired. Ms Alabi's predicament turns out to have been rather more complicated than the xenophobic presentation allowed. She had been in Britain legally and had wanted to leave, but was already too ill to travel. Her lawyers argued that the priority status she was denied reflected an overzealous interpretation of rules designed to discourage "health-tourism". Her death proves them tragically right. There were two people on the heart transplant list which Ms Alabi applied to join: would it really have overburdened the system to add a third?

The circumstances of Ms Alabi's case may be too unusual for it to offer a legal precedent. But it certainly stands as a warning of the human consequences of rushing to judgement and expecting migrants to fit into neat classifications. The line between legal and illegal may not always be easy to draw. And in cases that are not clear-cut, it behoves a country with Britain's wealth and tradition of fairness to err on the side of generosity.

It is no coincidence that the same people who find it hard to understand that ministers cannot know the unknowable figures for illegal immigration, also tend to regard immigration itself in a negative light. The facts, however, present a different picture. A report by the Blairite IPPR think-tank was the latest to find not only that the balance-sheet of Britain's relative liberal immigration policy is overwhelmingly positive, but that the country would benefit economically by declaring an amnesty to bring illegal workers into the legal economy.

Regrettably, the Government seems to have been scared by the released foreign prisoners fiasco into taking the opposite line. Rather than exult in the successful arrival of Polish and other workers from new Europe - said by some to be the largest migration to this country since the Huguenots in the 17th century - the order of the day is enforcement. At Prime Minister's Questions yesterday, Mr Blair insisted that the best way to combat illegal immigration was by means of identity cards and electronic border checks. Yet again, under pressure, he chose to share his critics' hard line rather than discredit them by force of argument.