A British officer's advice to the Indian authorities about how to resolve a stand-off with Sikh dissidents in 1984 was ignored by military chiefs who ordered a bloody raid which left up to 3,000 dead, an official investigation found.
David Cameron said the investigation by Cabinet Secretary Sir Jeremy Heywood found there was "absolutely no evidence" of UK government involvement in the raid on the Golden Temple complex at Amritsar.
The Prime Minister said the advice given by the British officer, who visited India and carried out reconnaissance at the Sri Harmandir Sahib site, was "not followed" and "was a one-off".
Papers released under the 30-year-rule revealed Margaret Thatcher's government agreed to a request from Indira Gandhi's administration to send a military adviser - with the documents indicating that an SAS officer had been sent.
The revelation prompted Mr Cameron to launch an investigation into whether the Thatcher government had been involved in Operation Blue Star, the Indian military raid which claimed hundreds of lives in June 1984.
The Prime Minister said: "Thirty years ago, a great tragedy unfolded at the Sri Harmandir Sahib in Amritsar. Many lives were lost and the scars in the Sikh community still run deep.
"So when documents came to light a few weeks ago raising the possibility that the then-UK Government was involved in the Indian army's operation that June, people were shocked, they were angry, and they rightly wanted answers.
"I immediately set up an inquiry to find those answers. I tasked the Cabinet Secretary with getting to the truth. He did so, and there are two main findings.
"One: around four months before the events, at the request of the Indian Government, a single UK military officer provided some advice. But critically, this advice was not followed, and it was a one-off.
"Two: there is absolutely no evidence of UK Government involvement in the operation itself.
"This conclusion has been made after a thorough search of some 200 files and over 23,000 documents."
"But let me be clear. I want the dialogue between this Government and the Sikh community to continue.
"British Sikhs have made - and continue to make - a vital contribution to our national life from serving in two world wars to running businesses and playing a massive part in our communities today.
"I never forget this. I am grateful for it.
"And I hope the manner in which we have investigated these dreadful events will provide some reassurance to the Sikh community, here in Britain and elsewhere."
Foreign Secretary William Hague t old the Commons that a British adviser travelled to India in February 1984, but his recommendation for a surprise assault using helicopters was not followed in the raid which resulted in a massacre.
Mr Hague said: "The Cabinet Secretary's report finds that the nature of the UK's assistance was purely advisory, limited and and provided to the Indian government at an early stage; that it had limited impact on the tragic events that unfolded at the temple three months later; that there was no link between the provision of this advice and defence sales and there is no record of the (British) government receiving advance notice of the operation."
The adviser travelled to India between 8-17 February 1984 "to advise the Indian intelligence services and special group on contingency plans that they were drawing up for operations against armed dissidents in the temple complex including ground reconnaissance of the site", Mr Hague said.
"The adviser's assessment made clear that a military operation should only be put into effect as a last resort when all attempts at negotiation had failed.
"It recommended including in any operation an element of surprise and the use of helicopter-borne forces in the interest of reducing casualties and bringing about a swift resolution."
In June that year the Indian army launched Operation Blue Star, which, according to official figures, resulted in 575 deaths. But Mr Hague said other reports suggested "as many as 3,000 people were killed, including pilgrims caught in the crossfire".
Mr Hague said the Indian plan "changed significantly" between February and June 1984, in response to a considerably larger dissident force and extensive fortifications within the temple complex.
Operation Blue Star was a ground assault, without the element of surprise or helicopter-borne troops.
"The Cabinet Secretary's report therefore concludes that the UK military officer's advice had limited impact on Operation Blue Star," Mr Hague said.
Further documents disclosed as part of Sir Jeremy's investigation reveal that the British adviser was told that the Indians "should not be able to pin any blame on us" if their operation went wrong.
The document states that it was "clear to the officer that the Indians had not given much thought to how they should root out the extremists, beyond applying the 'sledgehammer to crack a nut' principle".
The letter from Brian Fall, the private secretary to then foreign secretary Sir Geoffrey Howe, to Robin Butler, who was the principal private secretary at No 10, revealed the briefing given to the officer on his arrival in India.
High commissioner Sir Robert Wade-Gery told him: "If and when the Indians put the plan into operation and if it went wrong, they should not be able to pin any blame on us."
Shadow foreign secretary Douglas Alexander said "serious questions" remained: "The pain and suffering still felt by many about the tragic events of 1984 places a particular duty on the Government to provide what answers it can to address very genuine concerns."