Train passengers could be paying too much or risking penalty fines because ticket machines are so confusing, it is claimed.
The Office of Rail and Road (ORR) carried out a study with mystery shoppers which found that 13 per cent chose more expensive tickets than they needed.
A further six per cent were at risk of being fined after underpaying by selecting inappropriate tickets for their journey.
Britain's train operators should refund passengers who find they could have bought a cheaper ticket, the regulator concluded.
Two-thirds of the mystery shoppers did not see any information on the type of tickets they could not buy on machines, while 57 per cent reported that machines did not explain the times when off-peak tickets could be used.
Cases where the most appropriate ticket was not selected include one shopper who was "confused" as to whether to choose "any permitted route" or "not via Reading", while another found a ticket with a railcard was more expensive than without.
ORR director of railway markets and economics, John Larkinson, said: "Everyone travelling by train should be able to buy the most appropriate ticket for their journey."
Passenger groups claimed the report demonstrated that buying the correct ticket from a machine can be "far too hit and miss".
Anthony Smith, chief executive of transport user watchdog Transport Focus, said: "Ticket vending machines need to operate more like a human ticket clerk, steering people to buy the ticket that they want, rather than baffling them with too many options."
Consumer group Which? described the system as "a mess", adding that it was unacceptable that some passengers are "paying over the odds".
In December, the Department for Transport launched an action plan to improve rail ticketing.
It insisted that steps would be taken this year such as ending jargon, creating an online tool to explain restrictions and enabling passengers to find out when stocks of the cheapest advance fares are running low.
Rail minister Paul Maynard welcomed the ORR report, saying the ticket buying experience is "all too often complicated and hard to navigate".
He added: "Rail passengers must have confidence that they are getting the best possible deal every time they travel."
It was announced last week that a number of trials will take place in a bid to overhaul the fares system.
A spokesman for the Rail Delivery Group, which represents train operators, said: "Complex, decades-old government rail fare regulations make it more difficult for train companies to offer the right, simple options on ticket machines.
"Simplifying the number and types of fares in the system will let train companies provide customers with clear information and help them to make better informed choices."
Additional reporting by the Press Association.
