Britain said Tuesday it would investigate its involvement in India's 1984 Amritsar Golden Temple assault after previously top secret documents showed British elite forces played an advisory role.

Newly declassified government letters show New Delhi had requested British advice over a plan to flush out militants who had occupied the Golden Temple in Amritsar, northwest India -- considered Sikhdom's holiest shrine.

With the approval of then prime minister Margaret Thatcher, an officer from the elite Special Air Service (SAS) travelled to India and drew up a plan which was approved by India's prime minister Indira Gandhi.

It is not known how close the February 1984 plan was to the eventual deadly Operation Blue Star raid, which triggered a cycle of bloody revenge attacks.

Retired lieutenant-general K. S. Brar, who led the eventual June 1984 assault, said it was the first he had heard of it.

The raid on militants demanding an independent Sikh homeland left at least 500 people dead.

In retaliation, Gandhi was assassinated four months later by two Sikh bodyguards.

That triggered anti-Sikh riots in which thousands of people were killed, mostly in New Delhi.

Two letters released from the archives, both marked "top secret and personal", reveal details about the SAS advice.

The documents, declassified after 30 years, appeared on the Stop Deportations blog. The National Archives did not return calls.

One letter, dated February 23, 1984, is from foreign secretary Geoffrey Howe's private secretary to his counterpart in the Home Office interior ministry.

Howe wanted to warn home secretary Leon Brittan of possible repercussions of the planned attack.

"The Indian authorities recently sought British advice over a plan to remove Sikh extremists from the Golden Temple in Amritsar," it reads.

"The foreign secretary decided to respond favourably to the Indian request and, with the prime minister's agreement, an SAD (sic) officer has visited India and drawn up a plan which has been approved by Mrs Gandhi.

"The foreign secretary believes that the Indian government may put the plan into operation shortly."

The letter warns that the operation could trigger tensions in Britain's Indian community, "particularly if knowledge of the SAS involvement were to become public".

It says knowledge of the SAS officer's visit and plan has been "tightly held" in both India and London.

An earlier letter dated February 6, 1984 was sent from Robin Butler, Thatcher's principal private secretary -- the civil servant running her private office -- to Howe's private secretary.

"Thank you for your letter of 3 February about the Indian request for advice on plans for the removal of dissident Sikhs from the Golden Temple," it reads.

"The prime minister is content that the foreign secretary should proceed as he proposes.

"She will look forward to receiving a report on the adviser's visit."

Prime Minister David Cameron and Foreign Secretary William Hague were hitherto unaware of the letters, a government spokeswoman said.

Cameron has asked Cabinet Secretary Jeremy Heywood, Britain's most senior civil servant, to investigate further.

"These events led to a tragic loss of life and we understand the very legitimate concerns that these papers will raise," the spokeswoman said.

"The prime minister has asked the cabinet secretary to look into this case urgently and establish the facts.

"Any requests today for advice from foreign governments are always evaluated carefully with full ministerial oversight and appropriate legal advice."

Cameron's spokesman told reporters the investigation would also look into whether the documents should have been publicly released.

In India, Brar told news channel NDTV: "As far as I am concerned, Operation Blue Star was planned and executed by Indian army commanders.

"There was no involvement of anyone from the British government."