The Usha Multipurpose Co-operative Society - usually referred to simply as Usha - was established by Rekha Chatterjee 12 years ago, specifically for sex workers who were fed up with existing banks making it difficult for them to open an account.

It started out with only 13 members, but now has so many it is currently in the process of digitising operations and putting accounts into a computerised system.

"We want to increase our members to up to 20,000," Ms Chatterjee told BBC World Service's Outlook programme.

"We've managed to convince people that we're here to stay. Now, we have to bank on the trust that people have shown in us."

Good life

Because Usha is geared towards sex workers, it does not function like a normal bank. Many of its rules are more relaxed.

Customers can withdraw money by thumb impression, as some of them cannot write.

"But that's hardly a problem," said Ms Chatterjee.

"People are still maintaining their accounts, educating their children, and maintaining a good life."

Ms Chatterjee herself is from a poor family in Calcutta, and at home was beaten regularly.

She ran away and became a sex worker at the age of 25.

After eight years of prostitution in India's second-largest red light district, she became frustrated with the problems she and other sex workers faced when it came to looking after their money.

"Lawmakers, money lenders and local goons continuously harassed us - it was very hard to save money to plan for the future or save money for our children," she said.

"However, after the establishment of Usha, our lives started to change for the better. Now, with the bank in operation, our lives have been transformed completely and girls feel much more secure.

"They can withdraw money any time. Today, girls have a choice - which we just didn't have."

Expansion

Ms Chatterjee recently took out a loan from her own bank to build a house away from the red light district.

And she said she is "overwhelmed" by the respect people now give her.

"When I go to government offices, people stand up to greet me - they shout for people to bring me tea and a chair for me to sit on," she said.

"This is all a very new experience for me. Before, no-one would even offer me a glass of water, let alone to sit with them."

Ms Chatterjee is now set to retire from the presidency of the bank she built, but wants to establish a number of welfare projects before her term as president ends.

She said it is this which is fuelling her expansion of the bank to reach all over West Bengal.

"We'll talk to the girls all over the state and try to convince them their future lies with us," she said.

"We need to make them realise that this is their bank. However, we have enough credibility now, so it should be a bit easier.

"We want them to secure their future by building their own homes, educating their children and so on. We have to make them understand that this is for their welfare, and that we're always by their side."