New Delhi, Aug 29 (IANS) In more trouble for the West Bengal's ruling Trinamool Congress, the Central Bureau of Investigation (CBI) has written to Lok Sabha Speaker Om Birla for permission to prosecute three of the party's sitting MPs and one former MP in the Narada sting operation case.

"The agency has written to the Lok Sabha Speaker and they are awaiting for his permission to prosecute three sitting MPs," a CBI source told IANS but declined to reveal their names.

He said the agency has also sought permission to prosecute a former Trinamool MP.

Just before the West Bengal Assembly elections in 2016, Mathew Samuel, Editor and Managing Director of Narada news portal, broadcast a sting video in which a number of Trinamool Congress leaders were allegedly seen taking money on camera.

The CBI lodged an FIR in April 2017 following a court order, naming 13 Trinamool leaders and have interrogated many of them. The purported footage was also sent for forensic examination. The Enforcement Directorate is also probing the money trail in the case.

On Wednesday, the CBI grilled Trinamool MP K.D. Singh, Samuel and former TMC leader Mukul Roy in the case. Roy, after quitting the TMC had joined the BJP in November 2017. He was former aide of Mamata Banerjee, and had a fallout with her in 2015 when his name cropped up in the Saradha scam as well as the Narada sting operation. He was later suspended from the party for six years.