Sikh campaigners have demanded a full public inquiry to determine the extent of UK's involvement in the 1984 attack on the Golden Temple in India.
Regarded as one of the world's holiest shrines by Sikhs, the Golden Temple in the holy city of Amritsar, was stormed by Indian troops in June 1984 as part of Indian Prime Minister Indira Gandhi's controversial Blue Star operation, an attempt to remove dissident Sikhs seeking sanctuary in the temple.
The official death toll reported 492 militants, pilgrims and soldiers killed in the siege and the nation suffered some of the worst communal violence in its history.
The storming of the Golden temple led to the assassination of Indira Gandhi, when two of her own Sikh bodyguards killed her in revenge for the deaths of thousands of Sikhs.
Declassified letters released in January and lodged in the British National Archives, have suggested that an SAS officer was drafted in to help the Indian authorities stage the attack, under the orders of the then prime minister Margaret Thatcher, implicating the UK.
A letter from the Foreign Secretary's Principal Private Secretary Brian Fall dated 6 February 1984 refers to an "Indian request for advice on plans for the removal of dissident Sikhs from the Golden Temple".
It adds that the then prime minister, Baroness Thatcher, is "content that the foreign secretary should proceed as he proposes".
Following revelations of British involvement in the incident, the Sikh Federation UK and the British Sikh Council have issued an open letter to PM David Cameron, demanding answers.
In January, the prime minister commissioned cabinet secretary Sir Jeremy Heywood to investigate the claims and Foreign Secretary William Hague presented his findings which confirmed that the Indian government had sought advice from an SAS officer on how to carry out the siege, and were advised to carry out the raid as a last resort and utilise helicopters to minimise casualties.
However, Hague told MPs that the Indian government's campaign had "differed from the approach recommended" because there had been no "helicopter-borne element" during the raid and hence claimed that UK involvement had had a "limited impact" on the events at Amritsar.
In his Commons statement he said: "The then-government did send one military adviser to provide military advice on Indian contingency plans for an operation in the temple complex. However, this military advice was a one-off. It was not sustained.
"There was no other form of UK military assistance, such as training or equipment, to the Indians with Operation Blue Star. The actual operation implemented by the Indian Army differed significantly from the approach suggested by the UK military officer."
The foreign secretary added that while official figures put the death toll at 575, other reports suggested "as many as 3,000 people were killed including pilgrims caught in the crossfire. This loss of life was an utter tragedy. Understandably members of the Sikh community around the world still feel the pain and suffering caused by these events."
The Sikh Federation's Dabinderjit Singh alleges that Hague has misled Parliament about the details of the incident.
"Every single book (on the subject) says that actually helicopter gunships were used. They were used on the 4, 5, and 6 June 1984," said Singh.
"So that starts to put a different level of complicity in terms of what Parliament was told. So not only was the foreign secretary and the report that has been produced misleading, it was not truthful."
Singh further claims that tanks used in the siege were manufactured by a British firm, and representatives of Indian special forces had been offered SAS training in the UK.
"India turned it down. But the point is, what were the British doing offering that training? Why were they welcoming those special forces, who actually led the operation against the Sikhs?" asked Singh.
The Sikh groups' open letter to the government states: "The review has been perceived as a damage limitation exercise and the report produced showed no empathy for the thousands of innocent Sikhs killed."
The British Sikh Council's Gurmukh Singh added: "Unless there is admission of the truth, and the whole truth is revealed, and then there is reconciliation, you cannot draw a line under 1984."
A government spokesman said the UK government stands by the findings of the enquiry stating: "The (cabinet secretary's) report found that the nature of the UK's assistance was purely advisory, limited and provided to the Indian government at an early stage.
Singh has called for full disclosure of all documentation relating to the attack which he described as "one of the darkest episodes in Sikh history."