Bose's resignation came barely a day after he was released from jail on conditional bail after spending 75 days in CBI and judicial custody after his arrest by the central investigating agency in connection with the scam in November.

"I would like to inform that with immediate effect, I have resigned from my membership of the Rajya Sabha. I have also decided to resign from the primary membership of the party -
the Trinamool Congress," he said in a release.

His resignation prompted TMC national spokesperson and chief whip of TMC parliamentary party in Rajya Sabha Derek O'Brien to allege that Bose had been put "under tremendous pressure".

"We have been saying for some time now that he (Bose) has been put under tremendous pressure by the party in power at the Centre. He was released on bail yesterday. He resigned today. We are happy that he got 'freedom' yesterday," he said in a statement put up on TMC's website.

Party Secretary-general Partha Chatterjee said there was pressure on Bose to resign from the party and Parliament.

"Maybe pressure of BJP, maybe pressure of a news channel," was Chatterjee's cryptic reply when quizzed by newsmen in Burdwan at a programme.

Bose, editor-in-chief of a Bengal daily and former editor of Trinamool Congress mouthpiece 'Jago Bangla', said it was during his days in custody that he understood "politics is not my cup of tea".

Pressure from his family had also shaped his decision. "There has also been a lot of pressure from my family, specially from my mother and my wife which finally made me take the decision," the release said.

Bose thanked TMC supremo and Chief Minister Mamata Banerjee for making him an MP. "I would also like to express my deepest gratitude and thanks to all persons who have stood by my family during these extreme difficult time," he said.

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