Margaret Thatcher's government secretly considered acquiring chemical weapons amid fears that Britain had no answer to the Soviet Union's vast chemical arsenal, according to newly released official files.

Publicly ministers maintained that they had no plans to restore the UK's chemical warfare (CW) capability which had been voluntarily relinquished in the 1950s.

However, files released by the National Archives at Kew in west London show that, behind the scenes, Mrs Thatcher suggested the government could be considered "negligent" if it did not build its own chemical arsenal.

A Ministry of Defence paper from 1984 underlined the scale of the threat with an assessment that the Russians had more than 300,000 tons of nerve agents alone.

In contrast, the United States - which was the only Nato member to possess a CW capability - had an ageing stockpile of just 31,000 tons which was not actually declared to Nato.

Meanwhile, a Home Office working group calculated that a Russian CW attack by just three aircraft on Gatwick Airport would leave 16,350 dead and 29,000 injured while a similar attack on Southampton dockyard would kill 33,350 and leave 42,000 injured.

A note of a meeting of senior ministers and defence chiefs held on August 8 1984 - marked "SECRET: UK EYES A" - reported a warning by defence secretary Michael Heseltine that the lack of an retaliatory CW capability was a "major gap" in Nato's armoury.

"Reliance on a nuclear response to chemical attack lacked credibility," he warned.

Foreign secretary Sir Geoffrey Howe said public opinion needed to be brought "gently to a better and wider perception of the imbalance between Soviet and Nato capabilities in chemical warfare, while avoiding an upsurge of alarm".