The rail regulator has written to seven online third-party train ticket sellers to express concern that they are not being upfront about booking fees.
The Office of Road and Rail (ORR) said some retailers “are not as transparent as they need to be” over their fees.
It added that it wanted to make sure “that drip pricing does not undermine consumer confidence”.
The Department for Business and Trade has done a consultation into price transparency, including drip pricing.
The phrase drip pricing refers to the practice where a consumer is shown an initial price for a product or service, but then extra fees are added before the final cost is revealed.
The ORR’s review looked at how to prevent consumers paying more than they expect to because of the practice.
It analysed the ticketing websites and apps of 19 third-party sellers.
The ORR found 12 of the companies charged booking fees, and seven of these did not include their fees in the upfront price.
The regulator said booking fees ranged from 45p (per ticket) to £6.45 (per transaction) and finder’s fees were between 10-15% of the saving made on a split ticket.
The 12 sites found to charge booking and/or finder’s fees were: MyTrainTicket, Omio, Rail Europe, Railboard, Raileasy, Sojo, Split my fare, Train Hugger, Trainline, Trainpal, Trainsplit and TrainTickets.com. However, the ORR is not naming the seven companies it has written to.
Stephanie Tobyn, the ORR’s director of strategy, policy and reform, said its review “highlights that some online retailers are not as transparent as they need to be when it comes to how they display or provide information on additional fees”.
The ORR wants retailers to make sure customers have “readily available, transparent and accurate information about fees” before they start their booking.
The regulator said it had set out its concerns to the seven companies and asked them to make improvements. It has requested responses from the firms next month.
The review also looked at 21 train companies, who are not permitted to charge ticket booking fees although they can charge certain fees such as postage costs for ticket delivery.
Alex Robertson, chief executive at the independent watchdog Transport Focus, said: “Passengers should not be left in the dark about the cost of their ticket.
“Online retailers must provide passengers with clear, accurate information upfront so they can make an informed choice.”
Government research into drip pricing earlier this year found the practice was “widespread” across a range of industries, including air and rail travel, hospitality firms and the entertainment industry.