When asked in a television interview if Britain's immigration and asylum system made things more difficult for officers than in other European countries, Sir John warned: "I think there's a general acceptance that the borders we've got are porous, and that more steps are needed in relation to that."

But it's not only terrorists who test our border security, says Sir Timothy Garden, former assistant chief of defence staff and now visiting professor at King's College London, but international criminals, drugs and people traffickers. "There is a question of whether we need to put more efforts into securing our borders, but not to keep out refugees. Given the nature of this terrorist threat and the resources they have access to, they have a choice of legal ways of entering. I would be very surprised if asylum seekers posed a major problem. We've seen that people with US citizenship can be terrorists. I'd be more concerned about those who come in as tourists, students or employees."

Indeed, because the UK does not operate a registration system for foreign visitors, once someone has entered the UK legally, they have far more freedom than in other countries. But asylum seekers, who make up a tiny percentage of the 100 million international arrivals to the UK annually, are by their very nature exposed to official scrutiny more than those who enter by other means - from the moment that they are fingerprinted and photographed for their asylum registration card (ARC). Along with all visitors, asylum seekers claiming at port are also checked against the computerised warnings index, which is compiled by the security services and lists all potential immigrants about whom they have concerns.

However, because between 80 and 90% of asylum applicants provide no travel document or other proof of nationality and identity, this index is arguably of limited worth. In addition, because more than half of asylum seekers claim asylum once inside the country, any increased security at ports would be of questionable value.

Peter Bergen, fellow of the New America Foundation in Washington DC and the author of one of the most authoritative books on Bin Laden, is equivocal about the suggestion that exploitation of asylum processes is an established al-Qaida modus operandi. "People have claimed asylum and subsequently been arrested under anti-terrorist legislation but I don't know if it's the case that this is a serious problem, and there haven't been that many people arrested on terrorist charges in the UK to make a wider point." Anybody setting out to commit an act of terrorism nowadays is highly unlikely to claim asylum in order to enter a country, he adds. "They would come in as a legitimate visitor. They don't want to draw attention to themselves."

Jane Corbin, BBC Panorama journalist and author of The Base, an investigation into al-Qaida's methods, says that while terrorists have on occasion exploited asylum arrangements, further extrapolation is meaningless. "In Germany, one of the key suspects in the Hamburg al-Qaida cell was an asylum seeker, and it's believed that he was sent there to get himself into the system. Al-Qaida is adept at using the legal systems of the countries they wish to infiltrate, and at moving on when those systems are tightened up. In the early 90s, for example, we saw some groups moving from Paris to London following a clampdown by the French interior minister."

But with the spotlight on the asylum issue in this country, says Corbin, it is highly unlikely that any terrorist group would continue to exploit this route. "It's a convenient charge and a very dangerous one," she adds. "When people exploit the system, we should say so, but we should beware of sweeping generalisations."

5 Most asylum seekers are really economic migrants

"We do the jobs that you can't bear to do. They are very hard jobs, with long hours and low wages. I am a teacher. I have skills and qualifications but I can't get decent jobs, so I get up early and clean floors." Neka fled Sierra Leone with her husband in 1996, and both were subsequently granted full refugee status. She works as a cleaning supervisor, and people confide in her.

"Yes, there are people who come here to get better jobs, and maybe if the government made it easier to get a work visa it would ease the problem of asylum. These people have to be treated differently from those who have real difficulties in their countries and are in fear of their lives." It's hard, says Neka, because the media confuses the reasons why people come here. "In most of these countries there's a lot of suffering. They come here and become an important person, sending money back to their families." (IMF figures suggest that developing countries receive more than $60bn (�37.6bn) a year in such remittances, which is $6bn more than net official aid from OECD countries.)

"People are desperate and if claiming asylum or being an illegal is the only means of doing it then they will." It is impossible to know how many people view claiming asylum purely as a gateway to the relative riches of a job in Britain. But is it the case that the system is being deluged with bogus applicants? How many people seeking asylum have a plausible claim under the Geneva convention? Including appeals, around 40% of cases decided last year were officially recognised as in need of protection. Many campaigning groups argue that the requirements for full refugee status are becoming increasingly hard to satisfy.

They also note that 15% of applications were refused on purely procedural grounds, for example failing to complete the statement of evidence form, in which applicants have to outline, in English, their reason for seeking asylum, within the required 10-day period, or failing to turn up for interviews. Noting the difficulties faced by a traumatised non-English speaker struggling to adapt to a foreign environment, some take this figure to suggest that not all rejections relate to the substance or credibility of a claim.

And if the reasons why people come here are mainly economic, then why do the patterns of refugee migration mirror political strife in the countries of origin? Figures show that in the mid- 1980s the majority of refugees came from wartorn Yugoslavia, while in the last quarter most came from Iraq, Afghanistan and Zimbabwe. Others argue that those who apply once they are inside the UK are necessarily bogus. Surely someone fleeing persecution would make it their priority to secure their right to remain in a safe haven as swiftly as possible. But 65% of positive asylum decisions are given to these applicants and there are arguably valid reasons why people who are travel-weakened, confused, traumatised or fearful of further persecution might delay contact with the authorities.

For Nigel Harris, professor of economics at UCL, it's a no-brainer. "Economic migrants want to work. Asylum seekers aren't allowed work. If your motivations for coming to Britain are purely economic then you get the fare, come in illegally and find a job." Harris also believes that many refugees don't go through the asylum system until they have to. "People come here fleeing persecution. The easiest thing to do is to fade into London, get a job and pull your life back together. Why would you want to claim, when you are forbidden to work and get sent out to some rural backwater where the locals throw stones at you?"

A recent government white paper estimated that the number of illegal migrant workers - which includes those seeking asylum - could run into hundreds of thousands and noted that the problem was particularly severe where the availability of work is greatest, in lower wage employment sectors, such as catering, cleaning and hospitality. The bare fact of working illegally doesn't necessarily prove that an asylum applicant is an economic migrant, points out Heaven Crawley, a senior research fellow at IPPR specialising in the economics of migration. Nevertheless, she concedes that as long as other legitimate migration routes remain blocked, some people will resort to claiming asylum to try to secure entry for economic reasons. And there is no shortage of work in Britain's grey economy.

"There is a huge amount of illegal working at the moment because we have a chronic labour shortage. Although an estimated 175,000 work permits were issued last year, these were mainly for highly skilled workers. There are no permits aimed at people who could work in low-paid, low-skilled jobs, and we know that there are lots of illegal workers in these areas." It has been argued that these workers are undercutting the domestic labour force, adds even the lower-paid end of the British workforce isn't prepared to undertake this kind of work.

"Whether and how to provide further entry routes for low-skilled workers, and whether that would discourage those migrating for economic reasons from claiming asylum, remains a significant policy issue."

· The facts:

85,865 applications for asylum to the UK in 2002

8,100 or 10% of applicants were granted asylum in 2002

�1,050m spent on asylum support in 2001/02

�2.5bn the net fiscal contribution made by migrants - including asylum seekers - in 1999/2000

75% of new applications are determined within six months

12th the UK's rank in the EU in asylum applications received, compared to national population from 1992 to 2001

11th the UK's rank in the EU in total refugee admissions compared to national population from 1992 to 2001

72% of the world's 12 million refugees are given asylum by developing countries

300 schools have more than 10% of asylum seekers' children in England and Wales

�37.77 is received by a single adult asylum seeker in the UK

90% of UK employers want to take on refugees to meet skills shortages

3% of the world's population are defined as international migrants

20% of appeals are successful