Back in October, Virgin announced that it would continue to provide free WiFi through to the end of the year. Virgin said WiFi on London Underground would remain free for everyone throughout 2012 and, in 2013, Virgin customers would retain free access to the service. Virgin Media started rolling out WiFi in London's underground Tube stations in June. It started with King's Cross and Warren Street. Then Oxford Circus and Green Park were added, along with Victoria and Euston stations. Virgin originally promised free access throughout the summer and through to the end of the Olympic Games.
Not long after it announced that it was extending free access through to the end of the year, the company announced a deal that would see see Vodafone and EE (and, by default, Orange and T-Mobile) customers also enjoy free WiFi access in 2013. However, it seems you'll have to register if you want that free access.
Pocket-Lint reports that Virgin Media has begun contacting those that have used the service and is informing them that they'll need to register with your provider. If you're on Vodafone, you can do it via the My Vodafone scheme. Orange, T-Mobile and EE customers have a dedicated page for registering themselves.
What happens if you don't sign up? You'll have to pay. If you're not on Orange, T-Mobile, EE, Vodafone, or Virgin Mobile, or you fail to sign up, you're looking at daily, weekly or monthly passes that start from £2 per day.