Britain has said it would investigate its involvement in India's 1984 Amritsar Golden Temple assault after declassified documents showed that British elite forces advised the Indians on how to carry out the deadly attack.
The storming of the Golden Temple - considered Sikhism's holiest shrine - in Amritsar was one of the most violent episodes in the Indian government's battle against Sikh separatists.
Papers recently made public and posted to the Stop Deportations blog detailed a secret advisory mission to India by an unnamed officer from the Special Air Service, one of Britain's most elite military units.
It is not clear whether the Indians followed the officer's plan, but the allegation that Britain played a role in the raid is explosive because the attack was spectacularly bloody, leading to hundreds of deaths and a breakdown in communal relations across India.
In a statement, the government acknowledged that the raid, Operation Blue Star, "led to a tragic loss of life and we understand the very legitimate concerns that these papers will raise".
It said Prime Minister David Cameron had been caught unaware by the paper's content and had ordered an urgent investigation into the matter.
"Any requests today for advice from foreign governments are always evaluated carefully with full ministerial oversight and appropriate legal advice," the statement added.
A member of the Sikh religious organisation, Shiromani Gurdwara Pranbandhak Committee, Kiranjot Kaur, said she was staggered by the revelation and asked for a thorough probe.
"We need to know what transpired and what was the response of the British office to Prime Minister Indira Gandhi's letter to them. We would also like to know the details of the participation of the UK government in Operation Blue Star," Kaur said.
"Mrs. Gandhi always talked about a foreign hand, and we never realised that this could be the foreign hand she was talking about."
The Indian army's attack on hundreds of heavily armed Sikh separatists barricaded inside the Golden Temple in June of 1984 led to massive loss of life and a breakdown in communal relations across India.
Later that year, Gandhi was assassinated by two Sikh bodyguards, and the country was swept by a wave of anti-Sikh rioting.
The British officer's secret mission, disclosed in a February 1984 letter between two high-ranking officials serving ministers in the government of Margaret Thatcher, talked of an Indian request for British advice on how to remove the Sikhs, saying that a plan was drawn up and "approved by Mrs. Gandhi".
"The Foreign Secretary believes that the Indian Government may put the plan into operation shortly," the letter stated.