It was a phenomenon that even Mulder and Scully might have balked at.And shoppers certainly thought they were stuck in cult TV series The X-Files when a supermarket car park became a centre of spooky goings-on.

It was a phenomenon that even Mulder and Scully might have balked at.

And shoppers certainly thought they were stuck in cult TV series The X-Files when a supermarket car park became a centre of spooky goings-on.

As drivers looked on in amazement, at least 30 cars locked out their owners in a united display of mysterious mechanical madness.

Remote keys failed to operate, immobilisers rooted cars to the spot, and when flabbergasted shoppers stranded in Sainsbury's car park at Thetford tried to phone for help, not even their mobiles would work.

But it was not a case for the paranormal investigators to descend on Forest Retail Park.

Experts said they had traced the fault to Sainsbury's in-store transmitter, which switched itself to a different frequency, upsetting the car locks.

Apparently it was caused by stray radio waves, which can cut through a car's electromagnetic field in a split second.

As soon as the problem was discovered, the Radiocommunications Agency (RA), a government body responsible for the management of the non-military radio spectrum in the UK, shut off the transmitter.

An RA spokesman said: "The fault was traced to a local store. It just switched to the same frequency as these cars' remote locking, interfering with that wavelength and causing this problem. The RA shut it off and since then there have been no problems. It is not very common and is unlikely to happen again."

Sainsbury's staff spent most of the day stopping cars from parking in certain areas and pushing cars to less affected areas.

A spokesman said: "I have never experienced anything like it.

"The first we knew there was a problem was when more than one of our customers said that their remote controls were not working and they could either not get into their cars or they could not start their car engines.

"Shoppers were understandably confused by what was happening but my staff were great at helping them get their cars started and even giving out free cups of tea and phoning taxis and mechanics.

He apologised for any inconvenience caused, adding: "The problem was traced to an in-store communication system.

"The RA has done a thorough investigation and is satisfied that the problem has now been resolved."

Police were called in to patrol the car park while the RA was at work, to ensure thieves did not have a field day in the afflicted cars.

A spokeswoman for the Society of Motor Manufacturers and Traders, which supports the motor industry, said yesterday it was very strange that many cars had been affected at the same time.

The European Commission sets guidelines for radio frequencies and the level of electromagnetic field around a car, she said.

This "immunity level" stops radio waves affecting cars – which makes the Thetford incident highly unlikely.

"The cars would have to be right next to radio masts sending out stray signals," she said.

She said it was impossible for a portable radio transmitter to break the immunity level.

However, experts were unable to say what caused the transmitter failure – so perhaps it should be squirreled away in the X-Files after all.