AWE Aldermaston is believed to sit on a list of possible al-Qaeda terrorist targets.

Security agency MI5 is reported to have drawn up the secret list of more than 350 institutions considered to be at risk from terrorist attacks.

And terror fears have been fuelled by reports that aerial photographs of the nuclear bomb factory are readily available over the internet.

In the wake of September 11, MI5 is said to have listed key government buildings and installations on the "critical national infrastructure" list.

The sites have had their security profiles reassessed, and Aldermaston is among those believed to have been checked.

Meanwhile, on the Radio Four Today programme last week, a former anti-terrorist intelligence officer, Charles Shoebridge, expressed worries that attacks on installations could be greatly helped by having detailed photography of the sites, which can be bought for £45 on the web at getmapping.com

Mr Shoebridge said key locations for explosives to cause maximum damage could be identified.

Getmapping, based in Hartley Wintney, developed the Millennium Map, the first-ever high-resolution aerial survey of the UK.

The map is available free of charge at a low resolution and more detailed data can be bought on-line, but the company points out there are other sources available to would-be terrorists. There is no restricted airspace above 3,000 feet over the UK, many sites can be visited and photographed from within, satellite imagery is readily available from websites in Russia and the USA and photos can be taken from the perimeter of sites.

Tristram Cary, the company's managing director, said: "We are very conscious of the uses to which our data could be put, but Getmapping are not anti-terrorist experts.

"We have consulted the MOD and we are following their advice. If that advice changes then we will make whatever changes to our site are required to comply with national security requirements."