Foreigners are regularly refused entry to Britain because they are wearing the "wrong kind of shoes".

The most common method of weeding passengers out at airports is to look at what's on their feet, according to a former front-line immigration officer turned novelist, whose book set in the service is published next week.

Refusal Shoes, which takes its name from the practice, may be a comic work of fiction but some of the bizarre and haphazard methods used in the overstretched service it portrays are all too real, its author Tony Saint claimed.

"Any kind of shoe with tassels on and you are not getting in the country," revealed Saint, who resigned from the service three weeks ago.

Africans in particular fall foul of this unwritten rule of thumb, he said. "They may have a nice suit on but they always get the footwear wrong. Sometimes you look down the queue and you think, 'You'll get refused for those shoes'."

So-called bogey nationalities such as Nigerians get the greatest scrutiny from officers, as do flights from Turkey and Northern Cyprus, he said. "Do the Nigerians break the laws more than anyone else? Or is it because we spent so much time on them that we catch more of them?" he asked. "Flights from Ghana right next door don't get the same treatment."

Saint, who worked on the two toughest postings in the country for a decade, Heathrow and the Eurostar terminal at Waterloo station in London, said the whole system boiled down to whether officers "liked the cut of your jib".

He said: "Passengers have to satisfy immigration officers they are fit to enter the country, it is down to the officer's discretion. Some officers will refuse people to give themselves something interesting to get them through the day.

"I was the other way. I did everything to give them a chance. This thin blue line mentality is silly and old-fashioned and needs to be overhauled."

Saint even questioned the value of having immigration officers checking people coming into the country. "It is a waste of resources to have officers sitting at a desk and picking people out. Most at Heathrow are just in transit anyway.

"It works against the people we want in the country, and it is easier for the criminal and anti-social elements to get in. The immigration service needs to work more with other government departments, there is almost no coordination," he said.

Changing the way national insurance numbers were given out could cut the number of illegal immigrants overnight, Saint said. "If it was difficult, people wouldn't bother with Britain. At present the benefits agency don't check your status before giving out numbers, they simply hand them out to anyone who asks for one."

The whole immigration debate, Saint claimed, had been clouded by the sideshow of asylum. Instead, we needed a more honest and open immigration system, with an open door to people with the skills we need and tougher measures to shut out those we don't.

"So many people we see destroy their documents, so you have to take it on trust that they are who they say they are. The vast majority are given temporary admission.

"There is a big bogus moral element to asylum seeking," he claimed. "We must assume a story is true, and often it is, but the question remains whether we are giving asylum to the right people a lot of the time. It is very depressing when you see files on organised criminals who have come through under the guise of asylum. And we have had Rwandan war criminals claiming they were the victim of war crimes."

It obscured and unbalanced the system, he said. "I think we should give people the benefit of the doubt [without the recourse to asylum]. Everyone is on the make in one way or another anyway, and we shouldn't hold that against them."

Saint, 35, admitted there were bigots in the service. "If you are a bigot, it is a great place to work - because you can do something about it. But there is no more prejudice than there is at the BBC."

In fact, after a while, Saint argued, the job has the effect of making the whole "wash of humanity" feel the same. "The liberal notion of world fellowship goes. You don't care where they come from."

Refusal Shoes is published by Serpent's Tail.