The £60m Thames cable car currently running in the Docklands is costing £500,000 a month to run TFL has said.
The scheme, named the Emirates Air Line, was launched by the Mayor last June and is expected to cost £6m to run in its first year.
The Emirates Air Line enjoyed a surge in travellers during the Olympics and Paralympics, during which it connected two major venues at North Greenwich and the Royal Docks.
More than 1.7 million passengers have already taken the five-minute trip surpassing its first-year target of 1.3 million trips.
However, passenger numbers have fallen from 180,000 a week during the Olympics to as few as 300 travellers a day.
The cost of running the project was revealed in a freedom of information request.
TfL spent £2.296,000 in the first 20 weeks of the cable car’s operation - meaning its costs are on course to hit £6 million in its first 12 months.
The project also received £8 million from the European Commission.
Caroline Pidgeon, Lib-Dem transport chairwoman on the London Assembly, told the Evening Standard: “The Mayor promised Londoners that the Thames cable car would not cost a penny of taxpayers’ money, but in reality even in 10 years’ time the sponsorship deal will barely pay for half of its construction, let alone running costs.
“The Mayor’s false claims have also been exposed by the fact that too often the cable car is not even generating enough fare income to pay for its daily running costs.”