The CBI has asked Sovan Chatterjee, former Calcutta mayor and Trinamul leader who recently joined the BJP, to appear for questioning in connection with the Narada sting case.

The summons came on a day the central agency questioned Mukul Roy, former Trinamul leader now in the BJP, in the same case in New Delhi.

Chatterjee, who joined the BJP on August 14, has been asked to depose on August 31. The CBI has also asked Aparupa Poddar, a Trinamul MP, to appear in the same case on September 2.

Sources close to Chatterjee said he would “co-operate” with the agency. Chatterjee had earlier dismissed suggestions that he had switched to the BJP to save himself from CBI interrogation in the Narada case.

Mathew Samuel, the CEO of Narada news, had carried out the sting operation in 2014 and released the footage months before the 2016 Assembly elections in Bengal. In the footage, several Trinamul leaders and elected representatives were purportedly seen taking cash while promising favours in return.

“The Supreme court has directed the CBI to collect voice samples of those accused in the Narada sting case. We are in the process of summoning all those whose samples are yet to be collected,” said a CBI officer.