Up to one in 20 rail passengers could be travelling without a valid ticket on some routes, according to new research.
A sample of more than 23,000 South West Trains passengers found that as many as 5.5% avoid paying the correct fare.
The study, carried out for the Department for Transport (DfT), discovered cases such as forged tickets, child impersonation and using tickets at the wrong time.
This results in the franchise losing an estimated £25 million in revenue each year, the report found.
Ticketless travel was found to be most prevalent after 7pm on weekdays, with an average rate of 5.1%.
But due to the fare structure the most costly period in terms of lost revenue was the evening peak at £7.7 million, caused by a ticketless rate of 3.6%.
The overall proportion of passengers without a valid ticket was estimated as being between 2.7% and 5.5%.
In general, longer-distance and non-London service segments had lower ticketless travel than those dominated by short-distance journeys to and from the capital.
London Waterloo to Bournemouth/Weymouth saw just 1.5% of passengers without the correct ticket, compared to 4.6% on trains from London to Shepperton/Kingston.
The research was carried out over 15 days in October and November last year by Steer Davies Gleave, a technical advisor to the DfT in the re-letting of the South Western Franchise.