Trinamool Congress chief Mamata Banerjee has rejected Prime Minister Manmohan Singh's [Images] offer to join the Union Cabinet.

The Trinamool Congress Chief on Thursday ruled the possibility and said she was treating Singh's gesture as a political courtesy and nothing else.

''I am grateful to the Prime Minister for approaching me but we can't compromise our ideology by joining a Congress-headed Cabinet till the Congress continued to be in the company of Communist Party of India-Marxist for clinging on to power'', Banerjee said at a crowded press conference at her residence.

Mamata said it was a "casual" remark by the Prime Minister "and we are not taking it seriously".

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"I have worked with him in the Union Cabinet when he was the Finance Minister and I was the minister of state for Youth Services. I think that it was in that spirit that the Prime Minister told me he was missing me", Banerjee said.

The Trinamool Congress Chief said the Prime Minister's certificate to state Chief Minister Buddhadeb Bhattacharjee had shown that "his offer to me was merely a personal gesture whereas the actual Congress line came out in this misplaced praise of the chief minister who is the worst in India and whose government had failed on all fronts".

"Clearly, Dr Singh had to praise him because the Congress is enjoying power at the Centre with the help of the Left," she said.

Banerjee was of the view that Trinamool Congress was formed only because the Congress central leadership "failed" to understand that the state Congress had "surrendered" to CPI(M) "and sold out the party to the marxists".