Indian sex workers are being trained to identify counterfeit currency to prevent punters from conning them in the dimly-lit brothels of one of Asia's biggest red light districts.

Prostitution is illegal in India, meaning the country's estimated three million sex workers cannot complain to police if they are paid with fake notes.

But a campaign group known as the Committee for Indomitable Women has now begun a training programme in Kolkata's notorious Sonagachi red light zone, where an estimated 8,000 sex workers ply their trade.

"Most of them only come to know that they have been deceived when they go to deposit their earnings at the bank," Smarajit Jena, the group's chief advisor, explained to AFP during one of the training sessions.

"We are training sex workers through a machine with ultra violet ray to observe the differences between real and fake currencies."

The programme is being run to coincide with the start of the month-long Durga Puja festival, which begins on October 8 and which has traditionally been one of the most lucrative periods for prostitutes.

Some of those attending the workshop said they were often duped during the festival by one-off customers, and more regular clients would not dream of deceiving them.

"They take advantage of the dim lighting in our room and then slip counterfeit notes to us," said 30-year-old Swapna Mondal.

Her colleague Bharati De said it was crucial for the women to learn how to spot fake money as they could not turn to the authorities for help.

"Sex workers are afraid of going to the police station as they fear their complaint will go unheeded and they may be arrested," said the 40-yr-old, who has been working in Sonagachi for nearly two decades.

While police have not been involved in the training session, a senior officer in the eastern city acknowledged that counterfeit currency was a major problem.

Sumanjit Ray said that 521,155 fake bank notes worth more than 21 million rupees were seized from different parts of India last year.