The results of a government review into whether the SAS were involved in the 1984 Amritsar massacre are published today amid criticism from Sikh groups that it has been too narrowly focused.
Foreign Secretary William Hague is expected to make a statement to MPs setting out the findings.
David Cameron ordered the probe after documents disclosed under the 30-year rule suggested that a British officer helped the Indian authorities with plans to remove Sikh separatists from the city's Golden Temple.
In a letter to the Prime Minister, the chairman of the Sikh Federation Bhai Amrik Singh said he was "hugely disappointed" with the inquiry's "narrow terms" and a meeting with Cabinet Secretary Sir Jeremy Heywood had failed to assuage concerns.
"We are dismayed the terms of the review were only formally made available almost three weeks after the review was announced and only days before an announcement of the results of the review are expected in Parliament," he wrote.
"It appears the review has looked at a narrow period and not covered the period in the latter half of 1984 and may not have addressed some of the concerns raised by UK politicians in the last three weeks, e.g. threat of sanctions by India against the UK, Germany, Canada and USA towards the end of 1984 for sympathising with Sikhs in the Diaspora.
"From the outset you have emphasised the need for transparency, but the significant delay in sharing the terms and that they appear to have been changed for political reasons does not bode well with such assertions and your emphasis on the speed of the review."
It is not know whether the plan referred to in the documents from February 1984 was used by the Indian government.
The army did not mobilise until June, when hundreds of Sikhs were killed in Operation Blue Star.
A few months later the country's prime minister, Indira Gandhi, was assassinated by two Sikh bodyguards in what was widely regarded as a revenge attack.
A Downing Street spokeswoman said: "As soon as the PM became aware of these documents he tasked the Cabinet Secretary with establishing the facts about UK advice to the Indian government on its plans for operations at the temple complex in Amritsar.
"This is precisely what the review has done and the Foreign Secretary will set out the findings to MPs tomorrow."