Prime Minister David Cameron has ordered an investigation into official papers which suggest the British government of Margaret Thatcher helped India plan an attack against Sikh separatists in the Golden Temple at Amritsar in 1984.
The attack on Sikhism's holiest shrine angered Sikhs around the world who accused the Indian army of desecration. The death toll remains disputed, with the Indian authorities putting it in the hundreds and Sikh groups in the thousands.
The assault triggered the assassination of Indian Prime Minister Indira Gandhi, who ordered the attack, when two Sikh bodyguards killed her in revenge several months later.
Recently released British government papers from the time, publicised by Tom Watson, an opposition Labour party lawmaker, suggest Thatcher responded positively to an Indian government request for advice on planning the attack and sent an officer from the elite SAS special air service to help draw up a plan.
A spokesman for Cameron's office said on Tuesday the British prime minister had ordered an investigation as a result.
"These events led to a tragic loss of life and we understand the very legitimate concerns that these papers will raise," the spokesman said. "The Prime Minister has asked the Cabinet Secretary to look into this case urgently and establish the facts."
Jeremy Heywood, the cabinet secretary, is Britain's top civil servant. Cameron and Foreign Secretary William Hague had been unaware of the papers prior to their publication, the spokesman added.
Syed Akbaruddin, a spokesman for India's foreign ministry, said his government was only aware of the story from the media.
"We will take it up with our UK counterparts and seek more information," he told Reuters TV.