Britain planned to test a very virulent type of poison gas in what is now Botswana, previously secret documents from the UK’s colonial archive have revealed.

The plan for “practical trials” carried out on a “considerable scale” was first proposed by the Ministry of Aircraft Production in 1943. It was looking for an “isolated area” of about 26 square kilometres that was free of people, Britain’s high commission in Pretoria was told.

The Pretoria government quickly told the British that there was no suitable area within the Union of South Africa, according to the hitherto secret file marked “FORENSIC”, the codename given to the planned operation.

British officials then suggested that the Makarikari region of Bechuanaland would be suitable as a “chemical experimental area”. But facilities there were regarded as being too small. Moreover, one official noted, there was another disadvantage, namely “the presence of game including lions”.

The British turned their attention to the Makgadikgadi Pan, a large salt pan to the north of the then British protectorate. It would be preferable, they said, if at least some of the land chosen for poison gas experiments there was owned by the British crown.

In July 1943 a clearly nervous — and unidentified — British colonial official wrote to the UK ministry that was asking for a site to test its gas: “Your instructions have been noted and your letter destroyed by burning.”

An area south of Nata lake, to the east of the country, was chosen as being “eminently suitable” for the tests as there was no water within 25km, British officials noted. But by November 1943 they had still not decided where to test the poison gas. The trials were postponed “due to the proximity of the rainy season”, an official noted. So the plan would be postponed until the following year.

This is the last note in the file and it is likely that the plan never went ahead.

The information comes from the colonial archives — files that survived the purge of thousands of documents detailing some of the most shameful acts and crimes committed during the final years of the British empire, which were systematically destroyed to prevent them falling into the hands of post-independence governments.

The papers that survived the purge were flown discreetly to Britain where they were hidden for 50 years in a secret Foreign Office archive, beyond the reach of historians and members of the public, and in breach of legal obligations for them to be transferred into the public domain.

The archive came to light last year when a group of Kenyans who were detained and allegedly tortured during the Mau Mau rebellion won the right to sue the British government.

The Foreign Office promised to release the 8 800 files from 37 former colonies held at the highly-secure government communications centre at Hanslope Park in Buckinghamshire. —