The transaction will be done through the stock market via block deals.
BT -- which held 42.4 per cent stake in the joint venture initially -- has been consistently reducing its stake to focus on its core businesses back home.
The sale of the last tranche of shares to institutional investors is being done by Credit Suisse.
The share price of Tech Mahindra closed at Rs 882.35 on Thursday and the stake will be sold at a 7-8 percent discount to market price to attract investors.
Tech Mahindra stock has shot up 54 per cent in the year-to-date versus a 26 per cent gain in the BSE Sensex and a 1 per cent fall in BSE IT Index, giving a good opportunity to the BT Group to exit the company at an attractive price.
When contacted, a Tech Mahindra spokesperson declined to comment on the news. An e-mail sent to the BT Group did not elicit any response.
Although both the companies have maintained that the stake sale will not impact Tech Mahindra's business proposition with BT, the latter's share in the revenue contribution has been coming down.
Over the past few quarters, Tech Mahindra has reported that revenue from BT has been consistently falling.
For the quarter ended September 30, 2012, BT's business to Tech Mahindra was down by 3.6
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