A Congress stalwart, Mitra was the party president in the 90s. In 2007, he quit the Congress to form his own party which later merged with the Trinamool Congress.
Mitra, who described his rejoining the Congress as "home-coming", attacked the TMC-run government in the state on the two burning issues of atrocities against women and chit fund scams.
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"I have rejoined the Congress as I felt that the promises of the Trinamool Congress regime are not being fulfilled," he said.
"Congress has always been in my heart. It is a home-coming for me. I am very happy and grateful to Congress president Sonia Gandhi and Rahul Gandhi for giving me this opportunity," Mitra said at a programme in the presence of central party leaders Ambika Soni and C P Joshi.
Last week, he had resigned as Trinamool Congress MP from Diamond Harbour. Since late 2011, the veteran leader had been having a strained relationship with the Trinamool Congress.
The relationship strained further after he wrote a letter to Prime Minister Manmohan Singh over the mushrooming of chit funds in the state and sought his intervention.
The matter worsened after Mitra's wife, Shika, a Trinamool MLA from Bowbazar area, openly criticised Mamata Banerjee and Trinamool Congress, for which she was show caused and later on suspended. Talks of Mitra joining the Congress were doing the rounds for sometime after he recently got close again to the Congress leadership.
"Many are saying that I have betrayed Trinamool, but I want to say that I have betrayed none. I had written to TMC's highest leadership on November 7 last year expressing my wish to resign as party MP in January," said Mitra.
He said that women were no longer safe in the state. Everyday there are incidents of rape and gang-rapes.
"The CM has the time to visit Suchitra Sen in a hospital, but she didn't get the time to visit the family members of the Madhyamgram gangrape victim," Mitra said.