The Trinamool Congress on Tuesday night suffered a major split for the first time since its birth six years ago.
Party rebel leader Subrata Mukherjee floated a separate platform following differences with party chief Mamata Banerjee over the question of allying with the Congress.
The split comes barely two months before the Kolkata Municipal elections.
Mukherjee, the Kolkata mayor, has claimed the support of 10 of Trinamool's 59 members of Legislative Assembly and 11 of the 55 councillors.
"I am floating a platform, 'Paschimbanga Unnayan Congress Manch' and will form a political party in the coming days," he said.
Mukherjee, who hosted a dinner meeting for the MLAs and councilors, said, he will apply for a symbol and the party flag will have the tri-colour in the middle of the symbol.
Justifying his stance on allying with the Congress and other parties like the Samajwadi Party, Nationalist Congress Party, Janata Dal (United) and Party for Democratic Socialism, Mukherjee said Trinamool and the BJP with 27 per cent votes could not defeat the ruling Left Front in West Bengal.
"One-to-one fight is necessary to defeat the CPI (M)-led Left
Front," he said.
Earlier, rejecting Mukherjee's demand, Trinamool Congress chief Mamata
Banerjee said a tie-up with the Congress was not possible since it was running the central government with the Communist Party of India (Marxist)'s support.
She had accused the Congress of 'backstabbing' her party during the last assembly elections, for which the two parties had forged an alliance.
Mamata formed the Trinamool Congress after breaking away from the Congress in 1998.