Transport Secretary Alistair Darling said he was "extremely concerned" over allegations that undercover reporters smuggled weapons on board planes from Heathrow, Gatwick and Southampton.

A replica 9mm semi-automatic pistol was apparently taken aboard a flight from Heathrow to Edinburgh in the holdall of a Meridian Television reporter.

Another journalist is said to have carried a pair of scissors, hidden in a personal organiser, onto flights from Gatwick and Southampton.

This should never have been allowed to happen

Department of Transport spokeswoman

On Thursday Mr Darling met with Mike Hodgkinson, chief executive of BAA, which owns all three airports.

Afterwards a Department of Transport spokeswoman said: "The Secretary of State is extremely concerned about the allegations.

"This should never have been allowed to happen."

Mr Hodgkinson told the transport secretary BAA was investigating what went wrong and would "take the necessary action".

A BBA spokeswoman told BBC News Online: "We are not denying that something did happen but we are investigating the allegations made in the programme as a whole."

The alleged breaches in security were exposed by reporters working on the Meridian documentary called Air Of Danger, which is to be shown on ITV1 next week to coincide with the anniversary of the 11 September attacks.

The incidents come only a week after a man was arrested after trying to smuggle a gun on to a Ryanair flight from Stockholm to Stansted.

'References not checked'

Meridian said researchers working for the programme also managed to get jobs airside without their references being checked.

The programme makers said they had access to knives which could have been passed to a passenger boarding a flight.

Meridian's transport correspondent Mike Pearse said he had carried a Chinese-made imitation gun, wrapped up like a birthday present, onto a British Midland (BMI) flight.

BAA takes any breach of security seriously and we are investigating the allegations made.

BAA spokesman

It should have shown up on the X-ray machine it passed through.

Mr Pearse said: "I was amazed how easy it was to walk through Heathrow security with a gun undetected.

"It was fake but could have been real and that is cause for concern.

"It was very much the same story at Gatwick and Southampton."

He said: "What I did find staggering is the fact that we only took three flights and every time the system failed."

'Scandal'

Assistant producer Linsey Wynton said she had managed to carry a six inch pair of scissors onto flights from both Gatwick and Southampton.

She said that in both cases they were detected by an X-ray machine operator, but security staff failed to find them in her bag and allowed her to proceed onto a flight.

A BMI spokeswoman said: "BMI is shocked at the apparent lapse in security by the BAA, alleged to have taken place at Heathrow.

"The airline will be seeking reassurances and a full explanation at the highest level from the airport authorities with regards to security procedures and vigilance."

Two researchers also got jobs at Heathrow's Terminal One.

Meridian said both were told they could start work as soon as possible after their interviews, neither had references checked and one never even gave her address.

'Unmonitored'

Both were given temporary passes which meant they should have been escorted at all times.

The researchers said it would have been easy to smuggle a dangerous object to a passenger.

Graham Fowler, deputy general secretary of the British Air Line Pilots' Association (Balpa) said: "Yet again the security authorities have their eyes on the wrong ball."

He said security on the ground should be paramount.

We keep our security under constant review and we are actively seeking to improve security processes

BAA spokesman

A BAA spokesman said: "BAA takes any breach of security seriously.

"Improved security measures have been introduced since September 11 with a greater police presence and additional searching of passengers and baggage.

"We keep our security under constant review and we are actively seeking to improve security processes through new technology, working with government and research agencies."