BJP General Secretary J P Nadda said media reports that six front companies were used to transfer money to Singh and his family members "is an open-and-shut case of corruption".
According to the reports, Rs 1.6 crore were transferred into the account of Vakamullah Chandrashekher by the six "dud companies" between November 4 to 29, 2011 and in the same period Rs 1.75 crore were transferred in the accounts of Singh, his wife and Mandi MP Pratibha Singh and son Vikramaditya Singh.
BJP claimed this proves the money given to Singh was not "interest-free loan" but bribe received for granting favours.
"People sitting in high public offices who stoop to such levels of corruption deserve exemplary punishment. I demand that Singh should resign immediately as he has lost all moral rights to continue as CM," Nadda said.
The main opposition also asked Rahul Gandhi and Congress Chief Sonia Gandhi to intervene in the matter and sack Singh.