In August, the laboratory report said that the voices matched those of Virbhadra Singh and Pratibha Singh,' Dhumal said.

'The law and home department examined the matter therein, and in the meantime, certain people started asking the details about the case through the Right to Information Act,' he added.

As state Congress chief Kaul Singh Thakur got up to respond, Speaker Tulsi Ram said the issue could be discussed later and asked the opposition to give a notice for it.

This agitated the Congress legislators and they staged a protest walkout from the assembly.

The Himachal Pradesh police have filed corruption cases against Virbhadra Singh and his wife Pratibha Singh. The complaint against him is for misuse of official position and criminal misconduct when he was chief minister in 1989.

'Virbhadra Singh and his wife Pratibha Singh were booked Monday night by the state vigilance and anti-corruption bureau under the Prevention of Corruption Act, 1988. They were booked under sections related to misuse of official position and criminal misconduct,' Inspector General of Police (Vigilance) S.R. Mardi said.

He said they were booked on the basis of an audio CD released by Mankotia, a political rival of the Congress leader, in 2007. In the CD, Virbhadra Singh was heard referring to some monetary transactions on phone with former IAS officer Mahinder Lal, who is now dead. The CD also contained recordings of his wife, a former MP from the state's Mandi constituency.

According to a statement released by the vigilance bureau on the day of registration of the case, the couple has been booked on the complaint of Katwal on the basis of the CD released by Mankotia.