Veteran Congress leader from Assam and the party's former state President Ripun Bora quit the party and joined the Trinamool Congress on Sunday.
After his switch, Bora, a former Assam Minister and ex-Rajya Sabha member, tweeted: "From today I have started my new political journey!"
In his resignation letter to Congress' interim President Sonia Gandhi, he alleged that a section of Congress leaders in Assam have been in clandestine link with the ruling BJP leaders and Chief Minister Himanta Biswa Sarma.
He alleged that the party's interest and ideology are being compromised in favour of the BJP for some vested interests of a few leaders.
"During the past couple of years the way BJP, the symbol of communal and decisive forces is growing across the country and it is a serious threat to the democracy, constitution and secularism and economy of our country. But instead of fighting and unitedly and aggressively to prevent BJP at this critical juncture of the country," the letter reads.
It went on to say, "The leaders of this grand old Congress party at different levels are fighting each other for their vested interest. This has given BJP ample advantages to grow in every nook and corner in one hand and another, demoralizing millions of Congress workers of the country."
The Congress leader, accompanied by his wife and son, met Trinamool's national General Secretary Abhishek Banerjee in Kolkata and formally joined the party. Trinamool MPs Derek O'Brien and Susmita Dev, who was also in the Congress earlier, were also present.
Banerjee later tweeted: "Extending a very warm welcome to Shri Ripun Bora, a stalwart and skilled politician, who joined the AITC family today. We are extremely pleased to have you onboard and look forward to working together for the well-being of our people!"