Fares available include first-class and monthly tickets. The investigation, carried out by BBC South East, found some tickets, including a season ticket from Gatwick to the capital, were being advertised for a third of their retail price. Customers can use Bitcoin, a virtual currency, to buy them.

The investigation found that although the tickets do not have the correct metal strip that allows customers through barriers at stations, staff let users through anyway because the tickets appear genuine.

The group, who were not named in the investigation, told the BBC “no one should be ashamed of getting one over companies like Southern Rail”.