Kolkata, Jan 9 (IANS): Rebel parliamentarian from West Bengal's ruling Trinamool Congress Somen Mitra, who last year announced his decision to quit the party, will formally rejoin the Congress Jan 21, a senior Congress leader said Thursday.

Mitra, who represents the Diamond Harbour constituency in the Lok Sabha, will officially join the Congress at an event at the party's city headquarters Jan 21, state Congress president Pradip Bhattacharya said.

"This is the beginning of disintegration of the Trinamool. This development will pave the way for many more disgruntled leaders of the Trinamool to turn to the Congress ahead of the Lok Sabha polls," Bhattacharya said while making the announcement.

While the ruling party has been engineering defections from the Congress, this is the first time a big leader from the Trinamool would switch allegiance after the party came to power in the state in 2011.

Besides losing four of its legislators, the Congress has witnessed a large number of party workers and leaders crossing over to the Trinamool over the past few years.

Mitra, who has been vocal against his own party over the past few months, said he will be resigning as parliamentarian Jan 15.

A former state Congress president known for his organisational skills, Mitra was an arch rival of Trinamool chief and Chief Minister Mamata Banerjee, when both were in the Congress. He is regarded the prime reason for Banerjee's decision to leave the Congress and float another party in 1997.

Later, the arch rivals turned friends when Mitra joined the Trinamool in 2008, and a year later got elected as an MP on a Trinamool ticket.

However, Mitra, used to a position of eminence in the state Congress for decades, never got much importance in the Trinamool. That was not acceptable to the veteran leader.

Mitra's wife Shikha, a Trinamool legislator, who first raised the banner of revolt, has already been put under suspension.