The pilot scheme was intended to run throughout the duration of the Olympic Games last summer, but its popularity convinced the operator to extend the trial.
The service will be replaced with a premium version costing £2 for a day, £5 for a week and £15 for a month, but Virgin TV, broadband or phone customers can continue using it for free.
EE, T-Mobile, Orange and Vodafone mobile contract holder can also connect to the network free of charge thanks to a deal between the service providers and Virgin brokered in November last year.
WiFi is currently available in more than 80 Tube stations in the capital, with a further 28 to be added soon.
The free service is scheduled to end on January 29.