Singh, the then general secretary of Samajwadi Party, is accused of sending bribe money to win over three Bharatiya Janata Party lawmakers during the 2008 trust vote. Sources said Delhi police's Crime Branch will call him for questioning on Friday.
The decision to call Singh for questioning comes a day after arrest of Suhail Hindustani, who allegedly acted as a liaison between Singh and Bharatiya Janata Party MPs.
Hindustani was arrested on Thursday under Prevention of Corruption Act after seven hours of grilling by the crime branch about his role in the case.
On Sunday, the police had arrested Sanjeev Saxena, once considered close to Singh who allegedly delivered cash to the MPs to vote in favour of the United Progressive Alliance during trust vote, was arrested. The arrests and fresh investigations into the case have come after Delhi police was slammed by Supreme Court last week for its "callous" approach in this case.
Besides Singh, police is also expected to question Samajwadi Party MP Reoti Raman Singh and BJP MP Ashok Argal.
Questioning of BJP leader L K Advani's former aide Sudheendra Kulkarni, who allegedly was behind sting operation to trap the bribe-givers, is also likely to take place.
Cash-for votes: Suhail Hindustani arrested
Cash-for-votes: Saxena remanded to 3-day custody
Cash-for-vote: Deo justifies stand on Amar Singh
Skeletons out of UPA's closet
Man behind the cash-for-votes sting op speaks!