Transport for London (TfL) lost £22.3m in fares after giving free travel to people who live with its staff members, according to the Conservative Party.
The Tories said a Freedom of Information (FoI) request showed more than 8.4 million journeys were taken for free last year under the scheme.
The party's Free Ride report said the cost of TfL's "staff nominee pass" system could rise to £111m by 2020.
TfL described the Tory group on London Assembly's £22.3m figure as "nonsense".
The Freedom of Information data shows that between April 2014 and the end of March 2015, more than 3.5m of the free journeys were taken on London Underground, the Docklands Light Railway, or London Overground services. Another 4.8 million were bus or tram journeys.
Andrew Boff, Conservative Group leader, said: "At a time when the public sector is having to make money stretch further, there is no justification for individuals who do not provide an essential role in keeping London in operation receiving these types of benefit.
"It is about time TfL cracked down on waste, and got rid of this gold-plated perk."
But Transport for London said that the additional revenue it could have made would have been no more than £7m during the period.
"Were it to be withdrawn, it would result in immediate claims for increased pay and almost certainly lead to widespread industrial action," said a TfL spokesman.
The Rail Maritime and Transport (RMT) union said the staff benefit helped families of TfL employees to afford to live and work in London.
Spokesman John Leach said: "The Conservatives at the Greater London Authority should really push for proper funding on the Tube rather than feeding into people's resentment."