Margaret Thatcher gave her Indian counterpart Indira Gandhi Britain's full support in the immediate aftermath of the 1984 Golden Temple raid, according to private correspondence seen by the Guardian.
The then British prime minister sent a personal note saying that Britain supported India's unity in the face of demands for a separate Sikh homeland and disclosed that police were investigating threats against the safety of Indian diplomats.
The letter will cause further debate about Britain's role in the raid among the worldwide Sikh community and senior MPs across the political spectrum after it was disclosed on Monday that the Indian government had made an apparent request for advice from the SAS in the months leading up to the raid.
It will form part of an investigation launched by the cabinet secretary, Sir Jeremy Heywood, on the orders of David Cameron to determine the British government's actions over the raid on Sikhism's holiest site in Amritsar.
The Indian government says about 400 people were killed when Gandhi sent troops into the temple complex in June 1984 in the six-day Operation Blue Star. Sikh groups, which have called for an inquiry into the British role in "one of the darkest episodes in Sikh history", put the death toll in the thousands, including many pilgrims.
In what appears to be the first letter to Gandhi after the raid, sent on 30 June 1984, Thatcher wrote: "These have been anxious weeks for you, involving difficult decisions. I have followed closely your efforts to restore calm there, and I very much hope that the 'healing touch' for which you have called will open the way to a peaceful and prosperous future in that troubled region."
The letter, which is in response to two sent by Gandhi on 9 and 14 June, appears to show that the Indian prime minister had expressed worries that Sikh "extremists" could use Britain as a base. Thatcher wrote: "I well appreciate your concern about the potential security threat posed by extremists outside India. We are determined not to allow our traditional freedoms to be abused by those who seek to use violence for political ends."
In an apparent reference to death threats against Gandhi which had been reported in the British media, the UK prime minister who died last year wrote: "We have made sure the police are aware of these statements and they are investigating them."
Thatcher also reassured Gandhi that British police were "devoting considerable resources" to safeguarding Indian government personnel in Britain.
A few months after the letter was sent, Gandhi was gunned down by her own Sikh bodyguards in a claimed act of revenge. This triggered communal violence which led to the deaths of thousands of Sikhs across India.
Other documents in the file make clear Whitehall's interest in lucrative arms sales to India at this time. A secret Foreign Office briefing dated 22 June 1984, which was sent to Downing Street, stressed that British "commercial interests" in India were "very substantial. It it a large and growing market for both commercial and defence sales. British exports in 1983 exceeded £800m and since 1975 India has bought British defence equipment worth over £1.25bn," the document claims.
Cameron on Wednesday appeared to downplay the likelihood of an inquiry finding evidence that Britain was to blame for the raid. Labour's former deputy chairman Tom Watson suggested the British might have played a part in the assault on the temple in exchange for the Indians agreeing to purchase a fleet of helicopters in a £65m deal.
Watson said to Cameron: "On your Amritsar inquiry, instead of ordering the civil servants to investigate, why don't you just ask lords Geoffrey Howe and Leon Brittan what they agreed with Margaret Thatcher, and whether it had anything to do with the Westland Helicopter deal at the time?" Cameron dismissed any suggestions of a conspiracy.