The row over how much the British knew of the incidents 30 years ago threatens to derail Conservative party attempts to attract Sikh voters, who could play a major role in marginal seats in London and Leicester in any election.
Sikh groups in the UK criticised the scope of the inquiry and claimed it focussed on a very “narrow period”.
In a letter to Cameron, Sikh Federation UK chairman Bhai Amrik Singh said: “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.”