The BBC's Graham Satchell: "Human volunteers to test chemical weapons"

Bruce George, chairman of the Commons Defence Committee, said it would be "misleading" to say the parliamentary committee knew everything that was going on at the Porton Down base in Wiltshire.

On Thursday police began an inquiry into the death of a man during germ warfare tests there in 1953.

Mr George told the BBC : "It's too big for us to know, and, secondly, there are many things happening there that I'm not even certain ministers are fully aware of, let alone parliamentarians."

Bruce George: "There are many things happening there that I'm not even certain ministers are fully aware of"

But Defence Minister John Spellar said it was not possible to have total transparency at the research base.

He said: "They undertake enormously important work on defensive measures and obviously you don't want to publicise the details of all of that."

Gordon Bell says servicemen were tricked into the tests

Mr Spellar added: "We believe there are very good controls, there are ethical procedures in place, that have been evolved in discussions not just with our people but with medical bodies as well."

Tam Dalyell, MP for Linlithgow, first called for an inquiry into the base in 1968 when he went there as part of an investigation by the Committee on Science and Technologies.

Tam Dalyell MP: "In 1968 there were rumours things had gone wrong years before"

"We were given the impression there had been no fatalities, but there was concern about liquid Sarin and about the testing of uniforms and various other testings," he said.

He said he called for an inquiry following the visit, but was then "had up on a technicality and hauled before the Privileges Committee of the House of Commons".

He added: "The row was about a leak. But actually I hadn't intended to leak at all, because I thought that the minutes that were printed were public knowledge."

Health claims

A former serviceman, Gordon Bell, 61, has said his health was ruined by tests at the base when he worked there during the 1950s.

It has been alleged that scientists knew that some of the experiments they were carrying out could kill the volunteers.

The Technical Director of Porton Down, Dr Rick Hall, says the establishment is now very open

Ronald Maddison, 20, who was also in the armed forces at the time, died in 1953 during the Sarin tests.

In all, 300 former servicemen claim to have disabilities ranging from breathing difficulties to kidney complaints as a result of the tests.

The Ministry of Defence could face charges of assault and even corporate manslaughter.

Dr Rick Hall, the technical director of Porton Down since 1974, said: "The archives that are available suggest those going in for the tests were told what they were volunteering for."

Andrew Hosken explores Porton Down's past

He said that although the documents detailing Mr Maddison's death have not been published, the "essential details" have been printed many times since 1953.

But he said that despite the base's current "sinister reputation", it is "in fact very open", with the work it does published in technical literature.

"Clearly we don't seek to publish information that might help a potential aggressor, if they are looking to develop chemicals of their own," he said.

The UK has "not developed chemical weapons since the mid-1950s", he said, adding that the "sole work of the organisation" was to provide the UK and its armed forces with "effective protection, should these materials be used against us".

He said volunteers were still used but they were fully aware of the nature of the tests.