Since 2003 all staff who work in the "restricted zone" of terminals have had to undergo UK criminal records checks, but offences committed abroad are not covered. This means that the estimated "thousands" of foreigners who work airside at British airports may not have been fully vetted.

The Department for Transport (DfT) said checks of foreign criminal records are being considered as part of an independent review of personnel security announced in December. More thorough "counter-terrorist checks" are required for staff who work in particularly sensitive areas.

Tory shadow home secretary David Davis said the loophole would be a "disgrace" if it proved to be a case of "commercial interest trumping public safety". He called for immediate security checks to be carried out on all staff currently working airside regardless of the cost.

Mr Davis told BBC Newsnight: "It's astonishing given the history, and of course the risk on the airside of any airport in the United Kingdom. To know about this for five years and do nothing about it is doubly astonishing."

He added: "They should immediately carry out security checks on all people currently working airside. Anybody who fails should be removed and before anybody else is employed those security checks should be completed.

"And they should do that irrespective of cost and as fast as can be done. Otherwise it's worse than complacent, it's reckless."

Jim McAuslan, general-secretary of pilots' union Balpa, said people who could not pass criminal records checks should not be employed airside at all. He told Newsnight: "If it's good enough for pilots it should be good enough for anyone else that's working airside and these checks need to be carried out on everyone surely."

Transport Minister Jim Fitzpatrick was asked whether convicted EU terrorists could be working at Heathrow Airport. He told Newsnight: "What we're absolutely confident of (is) any individual who is working at our airports would have to go through the same screening process as anybody who wants to travel or anybody else who is working at our airports to make sure they are safe when they are working in that restricted zone area."

A DfT spokeswoman said: "All staff who work in the restricted zone of an airport are required to pass through the same security checks as passengers every time they enter it. Furthermore everyone must undergo a background check when applying to work in the restricted zone."