The London mayor's decision to accept sponsorship from a high interest loan firm for free travel on the Tube on New Year's Eve has been branded "shameful".
Posters advertising Wonga.com, saying "sometimes you need some extra cash" have been put up on the Tube network.
Labour London Assembly member Jennette Arnold said: "These legal loan sharks prey on the vulnerable."
But London Mayor Boris Johnson's spokesman said Wonga.com had made a "useful contribution" to Londoners.
Wonga.com's sponsorship will pay for free travel between 2345 GMT on New Year's Eve and 0430 GMT on New Year's Day on the capital's Tube, bus, Tramlink and Docklands Light Railway services.
Journeys will also be free on London Overground trains running after 2345 GMT.
Founder of Wonga.com, Errol Damelin, defended his company, saying: "Wonga is totally transparent about how we operate and how we charge for the short-term loans we provide."
"Our customers know exactly what they can expect to pay and they appreciate that clarity, which is reflected in our excellent customer satisfaction scores.
But Ms Arnold said: "Plastering London transport with offers of easy money at a time of year when people are at their most vulnerable is just shameful.
"I find it hard to believe that a legal loan shark waving money in people's faces at extortionate rates were the only company willing to provide sponsorship."
A Mayor of London spokesman said: "Wonga.com have made a useful contribution to the cost of providing free travel to Londoners on New Year's Eve.
"However the mayor would advise Londoners to always be careful if borrowing money and to be particularly cautious of punitive interest rates."
A Transport for London (TfL) spokesman said: "Wonga.com is making a contribution in support of free travel for this New Year's Eve."