Indian prime minister Manmohan Singh has come under pressure to resign because of his involvement in the allocation of coal reserves to private companies.

The main opposition Bhartiya Janata Party accuses Dr Singh of presiding over one of the biggest corruption scandals in Indian history.

The so-called "coalgate" scandal has cost Indian taxpayers more than $30 billion, and India's parliament has had to adjourn because of the uproar created by the issue.

"The PM is culpable for the delay in introducing transparent auctions," Ravi Shankar Prasad, the deputy leader of the BJP, told the upper house.

Between 2005 and 2009, 57 coal licences were issued to private companies.

They were given out on a nomination basis and not through a competitive auction.

India's auditor-general says as a result, the country missed out on an estimated $31 billion in revenue.

Dr Singh's government has been plagued by a series of corruption scandals.

But Dr Singh, who was in charge of the coal ministry from 2004 to 2009 and has been personally implicated in the mismanagement, has defended his actions.

He says his moves were within the rule of law - a stance his government backs.

"They (the opposition in parliament) are pleasing themselves with all kinds of preposterous demands," parliamentary affairs minister Pawan Kumar Bansal said.

"Their stand is not backed by any facts that is why they are causing a ruckus."

ABC/AFP