The Indian general in charge of the deadly raid on the Golden Temple in Amritsar in 1984 has dismissed claims that the UK colluded in the operation.
Lt Gen Kuldeep Singh Brar told the BBC that there was "no question" the Thatcher administration advised Delhi.
British PM David Cameron has ordered an investigation into the claims.
It comes after MP Tom Watson told the BBC he had seen declassified documents revealing the alleged involvement.
The storming of the Golden Temple was aimed at flushing out Sikh separatists.
The controversial raid, codenamed Operation Blue Star, outraged Sikhs around the world, who accused troops of desecrating the faith's holiest shrine.
Gen Brar, one of the architects of the operation, told the BBC's Newshour programme on Tuesday that it was the work solely of military commanders in India.
"I am dumbfounded and totally amazed," he said. "I do not believe that these documents could be authentic.
"As far as I am concerned the operation was planned and executed entirely by military commanders of the Indian army.
"There was no question of our getting help or advice from any foreign country - and certainly not from Britain - so as far as I am concerned [these allegations] appear to be like fiction," the 79-year-old former general said.
Gen Brar, himself a Sikh, has survived several assassination attempts including an attack in central London in 2012.
He said he "could not imagine" any scenario whereby covert UK support was supplied to the Indian prime minister at the time of the storming of the Golden Temple without his being unaware of it.
The Indian government says about 400 people were killed in Operation Blue Star in June 1984 - including 87 soldiers. Sikh groups dispute this figure and say thousands died, including a large number of pilgrims who were visiting the temple.
The Sikh separatists were demanding an independent homeland - called Khalistan - in Punjab.
Mr Watson, MP for West Bromwich East, said he had seen "top secret papers from Mrs Thatcher authorising Special Air Services (SAS) to work with the Indian government".
On Tuesday, the UK government said the cabinet secretary's investigation would try to establish the facts "as quickly as possible" but that he would also look at whether the papers should have been released or not.
Mr Watson cited two letters that have only just been released under the 30-year rule and published on the blog Stop Deportations.
One, dated 6 February 1984 from the prime minister's office, talks about the "Indian request for advice on plans for the removal of dissident Sikhs from the Golden Temple". It states that the prime minister is "content that the foreign secretary should proceed as he proposes".
Mr Watson also said the UK government appeared to have "held back" some documents and must disclose more information.
The Indian government has not yet commented on the reports.
India's main opposition BJP has called on the government to come out with the "facts", according to local media.
Operation Blue Star led to the assassination of Prime Minister Indira Gandhi, who was killed by her Sikh bodyguards in revenge.