They took place at the highly secretive Porton Down facility near Salisbury, despite there being virtually no information about the long-term health effects of even very low exposures to the lethal nerve agents.

As well as Sarin and VX, the tests involved Soman and Tabun. According to the documents, seen by The Times, the chemical weapons were used on open grassland and woodland.

The Ministry of Defence said last night that none of the tests was carried out on human beings or animals. The only people concerned were the military and scientists studying the effects of the chemical and biological tests.

More than 600 documents exist detailing the experiments in the 1960s: “The documents speak for themselves,” a ministry spokesman said.

The object of spraying Sarin, Tabun and VX gases over fields and woods was to see “how they would be affected by the climate and other atmospheric conditions”. Tests at sea were to check how chemical and biological agents might affect warships.

Hundreds of documents have been released by the Public Record Office in Kew about chemical and biological experiments at Porton Down in the 1950s and 1960s when human beings were used as guinea pigs. Those tests, however, involved small amounts of the gases and agents in laboratory conditions.

Those who “volunteered” from the military have claimed in recent years that they were told the experiments were about the common cold. Wiltshire police are investigating the case of Ronald Maddison, who died in 1953 after Sarin was put on his skin.

The Pentagon released the new files at the request of the Department of Veterans’ Affairs, which is lobbying for extra healthcare services for the 5,000 or so Americans involved in chemical and biological weapons testing.

It is not known how many Britons were involved, but claims for extra healthcare benefits by US servicemen could prompt similar claims in the UK — if any are still alive.

The test results are not known. It is also not known if the deadly agents were used directly on military personnel.

Crop-dusting was not the only method used to “weaponise” the nerve agents. The files explain how British and US military personnel used “rain-type” munitions and bombs.

The declassified documents acknowledge a far wider use of chemical and biological weapons than admitted by the British or US Governments. The disclosure could not come at a worse time for Tony Blair and President Bush, who argue that Iraq’s stockpiling of such nerve agents justifies a “regime change”. President Saddam Hussein used chemical weapons in the Iraq-Iran war in the 1980s and against his own Kurdish citizens.

Sarin, infamously used on the Tokyo subway by a terrorist group in 1995, causes a runny nose, watery eyes and breathing difficulty. It soon leads to vision problems, confusion, a coma, then death. VX causes vomiting and violent fits before death, all within about 15 minutes.

According to the documents, the project was ordered by Robert McNamara, the US Secretary of Defence, just after President Kennedy’s inauguration in 1961 and when Harold Macmillan was Prime Minister. The tests did not take place until 1967 and 1968, when Lyndon Johnson was US President and Harold Wilson was Prime Minister. The Porton Down tests were part of a British, US and Canadian programme. The US scrapped its biological weapons programme in the late 1960s.

• A report published yesterday said that lie detector tests were mostly inaccurate and had failed to catch a single spy, in spite of nearly a century of use. The report, by the National Academy of Sciences, urged America’s security agencies to stop believing that lie detectors, or polygraphs, are “magical mind-reading machines” and to acknowledge that they rarely produce reliable results.

  At present all employees of the CIA and National Security Agency are given a polygraph test. The FBI and Defence Department also test extensively.

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