Earlier this week, three guards working at Heathrow admitted taking thousands of pounds to allow people to travel on false passports.

Anthony Kreiger, 38, of Wesley Avenue, Hounslow; Faiz Rehman, 28, of Milford Road, Southall, and Pritesh Vaja, 22, of Braimar Avenue, Wembley were remanded in custody at Isleworth Crown Court, after admitting conspiracy to cause corruption.

How the Heathrow scam worked

One person checked-in on his own passport, receiving a valid boarding card He handed the boarding card to the passenger, and left the airport The passenger handed the boarding card and a forged passport - or no passport at all - to the corrupt guard at the gate He was waved through onto the plane

Police said the problem of such scams was growing - and could affect many airports other than Heathrow.

Detective Inspector Mark Pepper, of the Metropolitan Police's human smuggling unit at Heathrow, said: "Unfortunately I do not believe that this problem is limited to the companies involved.

"If people are attempting to [board planes] illegally it leads us to question their motive, especially since the tragedy of 11 September."

Terror fears

The Heathrow scam was discovered last May, when three Indian men were pulled off a flight bound for Canada.

If people are attempting to [board planes] illegally it leads us to question their motive, especially since the tragedy of 11 September

Detective Inspector Mark Pepper

They were travelling on stolen British passports - two bearing pictures of white men - yet all three men had walked past security guards hired by Initial Aviation Security, contracted by Air Canada.

The three security guards - two of whom were supervisors - were subsequently arrested.

Mr Pepper told the BBC he believed such scams posed serious problems to airlines.

"I believe it's a problem for the aviation industry. Various airlines do have a problem in this field.

'No security threat'

"Obviously the problem is dependent on the size of the aircraft, the locations where they fly to.

"Canada is a particularly attractive location, very much into human rights. I think it's been described as the land of opportunity, and people want to go there.

"Those particular locations and those airlines who fly to those locations are particularly vulnerable for this type of crime to take place."

The British Airports Authority (BAA), which runs most UK airports, said the problem had not been a security threat.

�6 wage

Kreiger, Rehman and Vaja said that since 1998 they had been paid up to �1,000 per person, to smuggle men onto flights.

So profitable had the operation been, that Kreiger was running a Volvo, a Cherokee and two BMWs - despite being on a wage of �6 per hour.

BBC Radio 5 Live reporter Christian Fraser said one of the problems was the low wages paid to people in very responsible positions.

The police praised Air Canada and Initiation Aviation Security for working proactively to deal with the problem.