In a decision that was expected but left political circles stunned, the Communist Party of India-Marxist (CPI-M) on Wednesday expelled Lok Sabha Speaker Somnath Chatterjee after he defied the party's appeals to give up the post ahead of a trust vote the government won.

He had been with the party for four decades.

In a unanimous move that was widely expected after the veteran parliamentarian took on many Communist MPs during the two-day debate, the CPI-M announced after a politburo meeting in New Delhi that Chatterjee had "seriously" compromised the position of the party.

A terse one-para statement said, "The politburo has unanimously decided to expel Somnath Chatterjee from the membership of the party with immediate effect. This action has been taken under article XIX, Clause 13 of the party constitution for seriously compromising the position of the party."

In Kolkata, the urban hub of the Indian Communists, CPI-M leader Biman Bose explained, "Violation of our party constitution is not allowed, is not allowed."

Chatterjee, who will turn 79 on Friday and continue as the speaker despite the expulsion, is one of India's most respected parliamentarians, having been elected to the Lok Sabha for the 10th time in 2004. He was also counted amongst the best speakers the lower house ever produced.

But his affability and long association with the CPI-M did not come to his rescue as seven members of the politburo met on Wednesday amid strong indications that serious disciplinary action was contemplated against him for daring to go against the party on the trust vote.

The CPI-M, the driving force behind the Left decision to withdraw support to Prime Minister Manmohan Singh's government this month, wanted Chatterjee to renounce the speaker's post and vote against the India-US nuclear deal and also against the ruling Congress-led coalition.

But in a daring display of defiance, Chatterjee refused and went on to say that the speaker's post was a constitutional one and had to be respected. He also tried to buy support from former West Bengal chief minister Jyoti Basu, one of only two surviving founder leaders of the CPI-M.

A furious CPI-M, a Stalinist party that brooks no indiscipline, then stopped pressing the issue but made it clear that action would be taken against him "at the appropriate time".

Realising what was in store for him, Chatterjee retorted, "Let them do what they want."

But a cautious Chatterjee resigned on Tuesday from the Sriniketan Santiniketan Development Authority (SSDA), a West Bengal body that looks after the development of Sriniketan-Santiniketan area excluding the Viswa Bharati University.

Wednesday's decision marks an unhappy end to Chatterjee's long association with the CPI-M that started in 1968 and where he rapidly gained stature, thanks mainly to his debating abilities that he used to mesmerise packed Lok Sabha.