The revelation of alleged SAS involvement in the planning of the 1984 expulsion of militants from the Golden Temple Amritsar was posted online on the same day as BBC News director James Harding's observation that Watergate-style scoops are as likely to be broken by bloggers these days as mainstream news organisations.

Phil Miller, an independent researcher and journalist behind the scoop, went to the National Archives in Kew seeking information on SAS involvement in Sri Lanka, a topic he has written extensively on since the government started to deport a larger number of Sri Lankans from the UK in 2011.

Instead he found letters revealing the then foreign secretary, Sir Geoffrey Howe, responding favourably to a request from Delhi for help to remove militants from the Golden Temple in Amritsar. Hundreds lost their lives in a six-day operation codenamed Blue Star. The correspondence was published on Monday on the Stop Deportations blog.

David Cameron has asked the cabinet secretary, Sir Jeremy Heywood, to conduct an urgent investigation into the decision by Margaret Thatcher's government to send an SAS officer to Delhi in 1984 to advise the Indian government on the expulsion of militants from the Golden Temple.

Heywood will want to examine why the papers were not marked sensitive and held back when papers from 1984 were released under the annual 30-year rule.

Miller co-founded the blog two years ago aiming to highlight the post-colonial role the UK has played in ongoing conflicts around the world.

"A lot of people ask why asylum seekers are here, when in actual fact, our continued involvement means we have a responsibility to give them refuge," he said. He quoted Institute of Race Relations director Ambalavaner Sivanandan's remark that "we are here because you were there" to further illustrate the point.

Miller referred to the apparent hypocrisy of Indira Ghandi whom, the letters show, was highly critical of the involvement of British intelligence personnel in Sri Lanka, only to request their assistance herself at a later date.

He is gradually piecing together the role of MI5 and the SAS in South Asia, using freedom of information requests, archives and interviewing asylum-seekers. Many have vivid childhood memories of seeing British military equipment in their home countries during conflicts. After discovering the Amritsar letters, he checked with a Sikh activist, who told him this was the confirmation they had waited 30 years to receive about the fateful event. It is often referred to as one of the darkest episodes in Sikh history.

The correspondence was released to the public at the National Archives in Kew under the annual 30-year rule. Anyone can request and view these documents in person, and in many cases online. The new wave of records released at the beginning of January have already revealed that Margaret Thatcher had planned to declare a state of emergency and use the military to transport coal at the height of the miner's strike.