Rail firms must "stop taking passengers for a ride" by charging first class fares on trains without first class carriages, Which? magazine has said.
Rail firms Southeastern and First Capital Connect (FCC) charge up to 63% extra for single first class tickets.
But Southeastern's high-speed London to Dover Priory route and FCC's Luton Airport Parkway to Wimbledon service do not have first class, Which? said.
Both the train companies have defended their first class ticket charges.
The magazine found that Southeastern was charging 30% extra for a single first class ticket on the high-speed route between St Pancras in north London and Dover Priory, but the service had no designated first class seating.
FCC charged passengers 63% more for single first class fares between Luton Airport Parkway station and Wimbledon in south-west London.
Although the trains had first class carriages they had been declassified and available to standard ticket-holders, the magazine said.
Southeastern told Which?: "A first class ticket will provide you with a choice of using the mainline first class or the high-speed service without the need to pay the supplement."
The magazine said this meant passengers were paying extra for a "standard-class, high-speed service, or a 40-minute slower first class service".
FCC told the magazine: "The ticket is available because you can travel first class for parts of your journey to Wimbledon if you choose."
But Which? found that to use that service passengers had to get off at St Pancras, walk to the Underground, take a 12-minute Tube ride to Vauxhall, and get back on the train again.
Which? editor Martyn Hocking said: "Rail companies need to stop taking passengers for a ride and only sell first class tickets if they can offer a first class service."
An FCC spokesman said: "It is perfectly feasible for the entire national rail journey from Luton Airport Parkway to Wimbledon to be carried out in first class.
"FCC tickets to Wimbledon all include cross London travel for people who decide not to travel on a direct service, therefore the suggestion that an additional fare needs to be paid is incorrect."