Mounting pressure on Uttar Pradesh Chief Minister Mulayam Singh Yadav ahead of the Assembly elections, opposition parties in the state on Thursday demanded his resignation following the Supreme Court order directing a preliminary inquiry by the Central Bureau of Investigation into the disproportionate assets case against him.

Reacting to the apex court order, leaders of Bharatiya Janata Party, Bahujan Samaj Party and Congress declared that it would definitely be an issue in the Assembly polls.

"Mulayam Singh should resign forthwith on moral grounds. It is not a political issue. It is a case of corruption and misuse of office by a chief minister and he must set an example," State BJP chief Kesri Nath Tripathi told PTI in Lucknow.

"Politicans must have clean image," he said adding, the CBI should complete the inquiry in time and submit its report.

BSP Legislature Party leader in the Vidhan Sabha Swami Prasad Maurya also made a similar demand and alleged that the Mulayam Singh government was neck deep in corruption.

"We hope the CBI inquiry will reveal the truth," Uttar Pradesh Congress Committee spokesman Akhilesh Pratap Singh said adding, it was a "serious matter" and the chief minister should immediately tender his resignation.

He said that it would be a big issue in the forthcoming Assembly elections in the state.

The Supreme Court on Thursday directed preliminary probe by CBI into allegations that Mulayam Singh and his family members have amassed wealth disproportionate to their known sources of income.

The direction came on a public interest litigation filed by Vishwanath Chaturvedi, a UP resident, who had alleged that the CM and his family members had amassed wealth by illegal means during 2001-05.