Sikh groups have criticised the scope of a government review into whether the SAS were involved in the 1984 Amritsar massacre.
David Cameron launched the probe - due to be published today - last month after documents disclosed under the 30-year rule suggested 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 chair of the Sikh Federation Bhai Amrik Singh said he was "hugely disappointed" with the inquiry's "narrow terms".
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"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 is not known 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.
Foreign Secretary William Hague is expected to make a statement to MPs today.