The report also shows that non-white Canadians are nine times more likely to be stopped than their white countrymen, and that non-white Americans were more than twice as likely to be stopped as they came off flights. It reveals that some officers referred to certain nationalities as "devious", "pushy" and "arrogant", relied on intuition and instinct to make their decisions, and described "rare occasions when, due to staff shortages, they had been instructed by chief immigration officers not to hold up any passengers at all".

But the researchers, who conducted the study at Heathrow and Gatwick from May to July 2005, said that discrimination did not play any part in the disproportionate number of non-white people being stopped. They argued instead that socio-economic factors played a role as poor people were more likely to seek work illegally or draw on public funds. They were also more likely not to be white. "There is a relationship between ethnicity and economics; this arises because of economic differences between countries and [within many countries] between individuals in different ethnic groups," the report concludes.

Ben Bowling, professor of criminology and criminal justice at Kings College London, disagreed with the report's conclusions. "The racial disproportionality is very large and very striking," he said. "The claim that it is about economics only goes so far. There is an assumption in the report that immigration officers can tell that black people are less well off than their counterparts just because of how they look. That is racial stereotyping." The Home Office failed to respond to requests for comment. Last week the government's immigration watchdog found that hostile media coverage of asylum seekers had affected the way immigration officers decided their cases.

The report, which contained quotes from interviews with immigration officials, shed light on factors that informed decisions. Officers said they might become suspicious if passengers said they had come to the UK on holiday but then had no idea of the sights they were going to see or where they were going to stay - unless they were Americans. "If Americans are vague about details of their visit ... this does not cause concern because immigration officers have found from experience that Americans are not necessarily very well informed about countries other than their own," the report said.

In some instances immigration officers generalised about nationalities being "devious", "difficult", "muddled", "naïve", "friendly", "pushy" or "arrogant". Because a two or three-week holiday tends to be the norm, "alarm bells start going" if passengers wished to stay for three to six months, the report said. "Officers can also be dubious if [poor] passengers are coming for a two- or three-week holiday, querying whether someone would 'save for two years' for such a short trip."

The way travellers are dressed also had a bearing. Those who "look the part" have less trouble such as "American ladies who've got loads of jewellery on ... and their clothes are really nice". But "young women wearing white stiletto shoes and short skirts ... may raise concern" because they "might be involved in prostitution".