Late British prime minister Margaret Thatcher had considered restarting Britain's chemical weapons programme at a cost of up to £200 million in response to Soviet threats, newly released Downing Street correspondence has revealed. Thatcher, by training a research chemist, said the government might be considered negligent for failing to acquire a "retaliatory capability" at the height of the cold war. The once-secret 1984 papers emerged from a Home Office file, released yesterday by the National Archives in Kew, which warned that airborne chemical attacks by Soviet aircraft on sensitive British targets could inflict massive loss of life. One paper estimated that up to 140,000 people could be injured and more than 20,000 killed if Liverpool's dockyards were hit. Britain had ratified the Geneva protocols in 1930, which banned the use of toxic gases and bacteria in war. But the treaty did not outlaw development or production of such weapons of mass destruction and permitted their use in retaliation. The communist bloc's expanding stockpile of nerve agents alarmed Ministry of Defence planners, who warned that there was no military response short of escalating to nuclear conflict. "Any significant step … to improve our retaliatory capability will involve serious political and presentational difficulties," a secret policy document stated. In February 1984, the prime minister attended a chemical weapons briefing. "Nato has no capability to retaliate in kind, although the Americans have a small stockpile," civil servants reported. "The threat of nuclear retaliation … is our only deterrent and is not credible in all circumstances." Another note stated: "Chemical warfare is likely to be an emotional issue and any increase in public awareness is … best delayed until the general public can be given credible guidance on protection measures." Among retaliatory options proposed were offering "practical support to the US administration by permitting the forward basing of US delivery systems and storage of chemical weapons in this country in time of tension or even in peacetime". "Finally, there is the option of acquiring an independent UK retaliatory capability," the note said. "The cost would be of the order of £100m to £200m. Such a decision would be a last resort should all other options fail." The file does not record the final outcome of deliberations, but a letter from Charles Powell, Thatcher's private secretary in Downing Street, recorded the outcome of a further, senior ministerial summit on chemical weapons in August that year. The following decision had been made, Powell said: "Modern nuclear, biological or chemical] equipment should be issued to servicemen and essential civilians in British forces in Germany and to some 140,000 servicemen in the UK with a Nato role. "The prime minister said it might be argued it was negligent of the government not to acquire a chemical weapons capability." Britain did not sign the chemical weapons convention - by which participating countries agreed to destroy all such capability - until 1993.