The Left Front lost control of Arambag Municipality in Hooghly district for the first time in nearly three decades after five CPI(M) and one Forward Bloc councillor switched to the ruling party.
The TMC also took control of the New Barrackpore Municipality after four of the five Congress councillors including the vice-chairman of the civic body switched sides.
"Five CPI(M) and one Forward Block councillor joined the Trinamool Congress today raising its strength to 12 in the 18-member Arambag Municipality," TMC general secretary Mukul Roy told reporters.
Congress suffered another blow as it lost control over the New Barrackpore Municipality with four out of five of its councillors including its vice-chairman Monoj Neogy deserted the party and joined the ruling Trinamool Congress.
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In the 20-ward civic body, TMC and Congress were running the board jointly.
Roy said TMC's strength increased from seven to 11 with the joining of four Congress councillors to the party.
The latest setback for Congress comes days after the party lost several municipalities including Sainthia Municipality in Birbhum district (which it had held for 15 years), Krishnagar Municipality and Coopers Camp in Nadia district to the TMC because of defections.
"Not only Congress, councillors from the Left Front are also joining us after being attracted by the development initiatives and good governance of the government headed by Chief Minister Mamata Banerjee," he added.
Since August, several opposition councillors in more than a dozen municipalities, mostly run by Congress, switched sides to the TMC enabling the party to gain control over them.
"This is a conspiracy by Trinamool to wipe out the Congress," said Pradip Bhattacharya, state PCC president.