British military advice had a "very limited" impact on India's 1984 Amritsar Golden Temple assault that left 500 dead, a government investigation found on Tuesday.

Foreign Secretary William Hague said the deadly raid on militants holed up in the northwest Indian temple in June 1984 was "entirely different" from plans suggested by an elite British military officer three months earlier.

British advice had therefore only a "limited impact on the tragic events that unfolded at the temple", Hague said.

"The UK's assistance was purely advisory, limited and provided to the Indian government at an early stage," he told parliament as he presented the findings.

The probe was ordered after newly-released documents revealed that an officer from Britain's elite Special Air Service (SAS) had advised New Delhi on how to flush out the militants from the temple -- considered Sikhdom's holiest shrine.

The officer told the Indian military to launch a surprise helicopter attack, according to the report. This plan was to be used only if negotiations had failed and was designed to minimise casualties, Hague said.

But the eventual assault, codenamed Operation Blue Star, "was a ground assault without the element of surprise and without a helicopter-borne element", Hague told parliament.

The raid on the militants, who were demanding an independent Sikh homeland, left at least 500 people dead and triggered a cycle of bloody revenge attacks.

India's then-prime minister Indira Gandhi was assassinated four months later by two Sikh bodyguards, sparking anti-Sikh riots in which thousands of people were killed, mostly in New Delhi.

Hague said there was no evidence that Britain had received advance warning of Operation Blue Star, or that the former colonial power had provided any further advice to the Indian military.

But he admitted that some of the defence ministry's documents on the assault have since been destroyed.

Britain gave the initial advice on the request of the Indian government because the two countries have an "important relationship", Hague said.

He dismissed suggestions from some British Sikhs that the advice was given in the hope that it would win defence contracts for British companies.

Retired lieutenant-general Kuldip Singh Brar, who led the assault, said last week that it was the first he had heard of any British involvement.

Four members of a Sikh gang were jailed in Britain in December for slashing Brar's throat in an attempted revenge attack as he walked through London with his wife two years ago.