Mukul Roy had quit Mamata Banerjee's party in 2017 (File)

Mukul Roy, the former number two of Mamata Banerjee, who joined the BJP last year, on Monday said the ongoing battle between the Centre and the state is "not good for the federal structure". The police officer at the centre of the controversy - Rajeev Kumar - should not have sat on protest along with the Chief Minister, he said.

The BJP has ripped into the Chief Minister since she sided with the Kolkata police chief, describing him as "among the best in the world" and holding an indefinite dharna to take on the Centre.

Various BJP leaders -- including Union Ministers Ravi Shankar Prasad and Smriti Irani -- have questioned why Ms Banerjee is going out on a limb for a police officer. "Circumstances indicate that this cop knows a lot and therefore has to be saved," Union minister Ravi Shankar Prasad said.

Mr Roy had been one of the leaders questioned in the Saradha Ponzi scheme that gypped lakhs of small investors of their savings, over which at least four lawmakers, including ministers, have been arrested.

Four years ago, he was also questioned by the CBI for what is referred to as the "Narada scam" after the website that conducted the alleged expose on a group of Trinamool leaders. Twelve leaders were accused of accepting cash in a sting by a journalist posing as a businessman and seeking favours from Trinamool.

"I am not guilty. I was interrogated when I was in the TMC. Investigation should go on and guilty should be punished," Mr Roy told NDTV.

There were apprehensions when Mr Roy quit the party in September 2017, peeved over the ascendancy Ms Banerjee was giving to her nephew Abhishek Banerjee.

In the months leading to his parting ways with the Trinamool, Mr Roy had been removed as the party's in-charge of Tripura. The party was seen as steadily making gains in the Left-dominated state till all its six legislators in the state switched to the BJP in August. He was last put in charge of Punjab.

Trinamool secretary general Partha Chatterjee had said, "Under pressure from central agencies, for his own self-interest, Mukul Roy was trying to hurt the party from within". There was suspicion that the leader was being provided immunity from investigation by the BJP in exchange for information that could compromise his party colleagues.