Sovan Chatterjee is an MLA from Behala Purba assembly constituency (PTI Photo)

Former Kolkata mayor Sovan Chatterjee, who was among Bengal Chief Minister Mamata Banerjee's closest aides, joined the BJP today, in a significant blow to the ruling Trinamool Congress in the state. Sovan Chatterjee is seen to be the BJP's biggest catch in Bengal after Mukul Roy, who said the exit would weaken his former party Trinamool immensely ahead of the 2021 state election.

"The BJP will win Kolkata corporation polls early next year," Mukul Roy told reporters as he welcomed aboard his former Trinamool colleague.

"He will now strengthen the BJP... Let me repeat that Trinamool will not even get the status of opposition party," Mr Roy said.

Sovan Chatterjee, a two-time Mayor, has always been close to Mamata Banerjee and was with the Trinamool Congress from its birth. Mr Roy said the 55-year-old played a big role in Mamata Banerjee's rise to the post of Chief Minister of Bengal. He was also known to be the fundraiser for the Trinamool.

In a famous episode immortalized on camera, Mamata Banerjee, at the opening of a children's swimming pool, gave Mayor Sovan Chatterjee a shove that had him falling into the pool. The incident, to many, revealed the Chief Minister's affection for Mr Chatterjee, whom she addressed by his nickname "Kanan".

In 2017, Mr Chatterjee was summoned by the Enforcement Directorate and the CBI in the Narada sting case. He was among the Trinamool leaders allegedly caught on camera taking cash from a journalist who carried out the sting.

Mr Chatterjee quit as Kolkata Mayor and state minister in Bengal in November after a falling out with Ms Banerjee following troubles in his personal life. Recent efforts by top Trinamool Congress leader Partha Chatterjee to mollify Mr Chatterjee and bring him back to the fold failed.

Mr Chatterjee is an MLA from Behala Purba assembly constituency. His friend and close aide Baisakhi Banerjee also joined the BJP.

In his three-decade career, Mr Chatterjee was a Congress councillor in the Kolkata Municipal Corporation in the 1980s but he broke away from the party along with Mamata Banerjee in 1998. She founded the Trinamool Congress the same year. He was elected as an MLA in 2011.

The BJP made deep inroads in West Bengal in the national election by winning 18 of the 42 parliamentary seats in the state, just four less than Mamata Banerjee's party.

The BJP intends to continue with its winning streak all the way to the state polls, and has managed to draw six Trinamool Congress MLAs besides one each from the Congress and the CPI(M).