Gordon Bown in a jam

An offical probe into the Heathrow crash has focused on the high-tech jamming device which shields Gordon Brown from terrorist attack.

When the Boeing 777 crashed on January 17 it passed just feet above the Prime Minister's official car as he was driven to the airport to board a flight to Beijing.

Inside the car is a jammer which broadcasts radio signals 100 times more powerful than a mobile phone.

The device is designed to block signals which MI5 say terrorists use to blow up remote-control bombs.

In the days after the crash, experts from the Air Accident Investigation Branch investigated one theory that these sophisticated jamming signals could have interfered with the plane's electronics and caused its mysterious double engine failure.

In recent weeks though, investigators have focused their attention on a fuel blockage as the cause of the crash.

A source told the Sunday Mirror: "The jammer was one theory early on, now they're looking at the fuel tank.

"At the critical moment the engines needed thrust there was a lack of fuel. Investigators believe this was most likely caused by a blockage or a fall in temperature causing the fuel to go solid."