The announcement, made late on Sunday night, put to rest weeks of anxiety and speculation of the growing distance between the two parties.

The decision was announced after External Affairs Minister Pranab Mukherjee, who also heads the state unit of the Congress, met Trinamool Congress chief Mamata Banerjee at a guesthouse.

“We will jointly contest the coming Lok Sabha elections in West Bengal against the ruling Left Front,” Mukherjee told reporters.

“Nothing could be better than this, we want a secular alliance at the Centre,” said Banerjee, flanked by Mukherjee and Keshab Rao, AICC office-bearer for Bengal. Adding that the alliance has the blessings of party chief Sonia Gandhi, Mukherjee underlined that it is inevitable in Bengal.

A formal announcement on the details of the seat-sharing is expected after leaders from both sides discuss the nitty-gritty. But the Congress has already announced that it would not field any candidate against Banerjee in the South Kolkata parliamentary constituency.

Rao, AICC member and in-charge of Bengal, who was at the meeting, said: “It is not a question of the number of seats we contest. But there are other factors too.”

At the meeting, Congress sources said, a strong faction demanded that in case of an alliance, the Congress should be given seats where the party has a chance of winning, apart from the traditional strongholds. In Bishnupur, Trinamool’s Madan Mitra — helped by the Congress which withdrew its candidate — won the seat, defeating CPM’s Iskander Hossain with a margin of over 30,000.