Leading article: The unseen workers who add to our prosperity

It is notoriously difficult to calculate the number of illegal immigrants in a country. And for a nation as diverse as Britain, it is impossible to say with any degree of certainty how many people are here unofficially. The Home Office's best estimate is 430,000. This figure was achieved by subtracting their official estimate of the "legal" foreign-born population from the total foreign-born population as recorded in the last census. This method of calculation is far from perfect, but at least it gives us something to work with. And even this scrap of information tells us something significant about Britain: that illegal immigrants offer a substantial boon to the UK economy.

It is important to refute one of the first charges always levelled by ill-informed xenophobes: the people in question are not collecting state benefits. Attempting to do so would reveal their existence. Nor are they starving in our streets. Most of them are clearly able to find food and shelter for themselves. We have to conclude that virtually all of them are working in the UK's black economy. It is easy to forget this fact, amid the constant demonisation of these people in the media as organised criminals, benefit cheats and terrorists. Rarely are they recognised as the cash-in-hand workers who make a quiet but significant contribution to Britain's prosperity.

As a study by the Institute for Public Policy Research points out today, most work in sectors like cleaning, care work, hospitality and food production. These are the low-paid, low-status jobs that British-born people shun. Without foreign workers - whether legal or illegal - vital areas of the public and private sectors would grind to a halt. Illegal migration is a good guide to where the gaps in our labour market are - and it is not in highly skilled sectors, whatever the Government's new points system for immigrants seems to suggest.

The IPPR study makes another interesting point. It estimates that allowing people at present living here illegally to join the official economy and pay taxes would boost government revenues by £1bn a year. That would clear the ballooning NHS budget deficit at a stroke - something for the Chancellor to consider. The alternative is to carry on as we are now, forcibly deporting people who are actively benefiting our economy. The costs of this alone should encourage us to look for a better way; forced deportations for all illegal immigrants in Britain would cost the taxpayer around £4.7bn.

The question of what to do about illegal migrants is not just an issue here in Britain, of course. A fierce debate is taking place in the US Senate over a Bill that would make life harder for America's 12 million undocumented workers. But America has a history of offering amnesties to such people, and that tendency could yet prevail. Here in Europe, Spain performed a similar regularisation programme last year - something that, tellingly, has boosted the Spanish government's tax take. This is the example we in Britain should follow.

There are moral, as well as economic, reasons for doing so. Tony Blair and Gordon Brown claim to be passionate about helping the poor of the developing world, but when a relatively small number of enterprising souls travel here in search of a better life, they allow the Home Office to treat them as a "problem". Of course, one of the most effective ways of transferring assets from the West to the developing world is not government aid, but remittances home from foreign-born workers.

Britain, it is true, does not owe the rest of the world a living. But when a small number of eager workers from abroad make it to this country - offering us as much as we offer them - we surely have a duty to treat them as friends, rather than common criminals. It is, after all, in all our interests.