Replica bombs were smuggled past security staff in hand luggage during a safety inspection at Britain’s second busiest airport.

Staff at Gatwick failed to identify artificial explosives carried by undercover transport inspectors from Brussels even though one device was allegedly identified as suspicious by X-ray scanners.

The device was apparently handed back to the purported terrorist because the person carrying out the screening did not realise what had been found, according to an airport source.

The shortcomings high-lighted by the European commission’s inspection this month will be tested again this week in a follow-up audit.

Sources at Gatwick claim the work of security staff is being hampered by the need to keep queues to a minimum.

Under rules introduced in March, BAA, the owner of Gatwick, can be fined up to £17m a year if passengers are consistently forced to queue for more than five minutes for security checks.

An airport source said: “Staff are under massive pressure to get people through queues and can easily miss things.

“In one case [during the recent inspection], a replica bomb was flagged up on screen, the bag searched and then handed back to the inspector because they didn’t know what it was. There is so much focus on looking for liquids that they seem to be forgetting the basics.”

The source said screening staff were also being distracted by virtual tests, known as threat image projection, which involve a computer superimposing an image of a banned item on the screen as baggage goes through. “They are spending so much time looking for these virtual items that they are not focusing on those in the real world.”

The incident is not the first time security at Britain’s airports has been found wanting. In 2000, inspectors managed to get through security at Manchester and Stansted airports with fake guns and bombs.

Experts claim that explosives can often be difficult to detect. “Because explosives are organic they are difficult to distinguish during the x-ray,” said Norman Shanks, a former head of security at BAA.

“Staff have to look instead for smaller parts, such as the timer, wire and detonator which can be very difficult to spot. We really need to be looking at a new generation of X-rays to help check for explosive devices.”

A BAA spokesman confirmed the inspection had taken place, but refused to comment on whether it had failed to detect replica bombs. “We regularly work with European Union and British government inspectors to rigorously test security processes and staff to continuously improve our procedures,” he said.

The European commission refused to discuss the findings of the inspection.

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