The Prime Minister's Office has given "instruction" to the Coal Ministry to help India's nodal investigation agency Central Bureau of Investigation (CBI) in its probe into alleged irregularities in allotment of coal mines without auction between 2006 and 2009.

"The Prime Minister himself wants that each and every aspect should be investigated. It is also a standing instruction by the Prime Minister's Office to cooperate in the inquiry," India's Coal Minister Sriprakash Jaiswal told PTI.

The Coal Ministry has appointed senior officials to help the CBI in its probe, he said.

Out of over 100 firms which were allocated coal mines without auction, the investigating agency has so far collected documents relating to 68 companies, according to the Coal Ministry sources.

The CBI is investigating alleged irregularities in mines allotment without auction between 2006 and 2009 after the case was referred to it by the Central Vigilance Commission.

The Coal Ministry was under the Prime Minister's charge during this period.

Jaiswal said, "In case any one is found guilty, stringent measures would be initiated against them."

Earlier, as per a leaked draft report, the Comptroller and Auditor General (CAG) had estimated a Rs 10.6 lakh crore loss to the exchequer on account of allotment of coal blocks during 2004 to 2009 without auction.

However, the government has denied it saying that the allocations were made in a transparent manner in consultations with the concerned state governments.