Defence lawyers for a prime suspect in 1993 bombings in the Indian city of Mumbai (Bombay) say they suspect he may have confessed under duress.
Abu Salem is one of India's most wanted men
Prosecutors lodged a sealed envelope, thought to contain the confession, at a court in the city on Monday.
Lawyers for Abu Salem say their client maintained his innocence when they last saw him on Sunday, and they would be surprised if he had changed his plea.
Abu Salem is also facing murder, extortion and kidnapping charges.
Abu Salem denies any part in the bombings, which killed more than 250 people and left 1,000 injured.
He and his companion, former Bollywood actress Monica Bedi, were arrested in Portugal in 2002 and extradited to India earlier this month.
Abu Salem's lawyer, Ashok Sarogi, told the BBC that he had been present when a sealed letter was given to the judge in Mumbai by India's main investigating agency, the Central Bureau of Investigation.
He said the covering letter suggested that it contained a confessional statement. It will be opened in court on Tuesday, Mr Sarogi said.
Mr Sarogi said confessions in police custody are accepted by the courts, but he wanted to find out under what circumstances any confession might have been obtained.
He said any confession obtained by force could be challenged in court.
Abu Salem is currently in CBI custody. A spokesman for the agency was unable to comment on reports that he had confessed.
There are at least another 25 cases of murder, extortion and kidnapping registered against Abu Salem.
He is also accused of terrorising Bollywood film stars and producers.
Authorities in India say Abu Salem, an Indian Muslim, is a key associate of Indian crime lord Dawood Ibrahim.
Hundreds were killed and injured in the blasts
Dawood Ibrahim is also suspected of involvement in the bombings, believed to have been carried out in revenge for the deaths of hundreds of Muslims in riots in 1992.
A Portuguese court sentenced Abu Salem to four-and-a-half years in jail in November 2003, for possession of forged documents and resisting arrest.
Monica Bedi was sentenced to two years for using false documents.