Citi's sale of the back-office unit to India's biggest software services exporter was set to be announced later on Wednesday, the source said.

A TCS spokeswoman said the company had called a news conference at its Mumbai headquarters at 0900 GMT and declined further comment. A Citigroup spokesman declined to comment.

Citigroup Global Services, the India back office unit, began as a business processing arm for Citi India in 1992 and expanded to serve Citi's global operations in 1998, according to its website.

The unit, which employs 12,000 people, operates out of seven facilities across Indian cities and offers back office services to Citi's consumer, corporate and global wealth management entities in 50 countries, the website showed.

In July last year, Infosys Technologies, India's No. 2 software services exporter, signed a $250 million outsourcing contract with Royal Philips Electronics and bought three of the Dutch firm's back-office centres.