The Communist Party of India (Marxist)-backed Centre of Indian Trade Unions on Thursday strongly opposed the proposal of Sterlite Group acquiring the remaining 49 per cent stake in Balco, saying the NDA government struck the previous deal for a "pittance" and the matter should be investigated.

Close on the heels of moves by the Anil Aggarwal-promoted Sterlite group to exercise its call option three years after it first acquired 51 per cent majority stake in Balco, CITU has now petitioned Finance Minister P Chidamabram against any further sale of equity.

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CITU president M K Pandhe asked Chidambaram to institute "a thorough probe into the shady deal between the National Democratic Alliance government and Sterlite."

He said any decision on the matter should be preceded by discussions with the trade unions.

Official sources had earlier confirmed that Sterlite group had intimated the government about the proposal to hike its stake and said the deal was yet to be concluded.

The three-year period, prescribed under the shareholders agreement for exercising the option, ended earlier this year.

Sterlite had acquired from the Centre 51 per cent stake in Balco for Rs 551 crore (Rs 5.51 billion) in one of the first strategic sales pursued by the previous NDA government.

According to CITU, the trade union arm of CPI(M), which extends crucial outside support to the United Progressive Alliance government, Balco was sold for a "pittance".