Former Indian Prime Minister Manmohan Singh has been summoned as an accused, along with five others, in a case linked to the coal block allocation scam that saw mining rights being assigned allegedly without any transparency to private firms.

A special CBI court has also named Kumar Mangalam Birla, chairman of aluminium manufacturing company Hindalco, and leading Indian industrialist and former Indian Coal Secretary PC Parakh among the six accused. They have all been asked to appear in court on April 8.

The court today rejected Central Bureau of Investigation's finding in its final report that there was no prosecutable evidence against anyone, reports our New Delhi correspondent.

Manmohan Singh, who was the prime minister of India for 10 years till his Congress party lost power in May last year, was reportedly questioned by CBI in January in connection with the allocation of a coal field in a coal block in eastern Indian state of Odisha to Hindalco.  At the time, Singh had held the charge of the Coal Ministry.

Hindalco, which is part of the $40 billion Aditya Birla Group, had first been refused the coal field it sought but the decision was later oveturned.  The company has denied any wrongdoing and the former prime minister had in a statement in 2013 defended the action.

The court had in December last year asked for Singh to be examined after CBI suggested it would like to drop the case against Hindalco.

The scam, dubbed in Indian media as "Coal-Gate", surfaced after the national auditor's report in 2012 questioned the government's practice of awarding coal mining blocks at a concession to companies without competitive bidding.

The Supreme Court had last year scrapped nearly 214 coal blocks allocated by successive governments over the past two decades.