Some rail companies are selling first-class tickets for trains that have no first-class seating, according to Which? magazine.
It said the Southeastern rail company was charging 30% more for a single first-class ticket on the high-speed route from London to Dover Priory, which had no first-class seating.
The magazine also said that the First Capital Connect (FCC) train company charges 63% more for single first-class fares between Luton Airport Parkway station and Wimbledon in south-west London.
Which? said this line had no first-class service but had declassified first-class carriages, where holders of standard tickets can sit.
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."
Commenting on this Which? said: "So paying 30% extra buys a standard-class, high-speed service, or a 40-minute slower first-class service."
FCC told Which?: "The ticket is available because you can travel first class for parts of your journey to Wimbledon if you choose."
Which? said: "When we checked for the most convenient way to use the first-class service you would have to get off at St Pancras (in north London), walk to the Underground, take a 12-minute Tube ride to Vauxhall at a cost of £4 (£1.80 with an oyster card) and get back on the train again."
Which? editor Martyn Hocking added: "Rail companies need to stop taking passengers for a ride and only sell first-class tickets if they can offer a first-class service."
In a separate article, Which? found rail passengers could save money by buying two tickets instead of one for a single, same-day journey.
The practice of "split ticketing" could cut the cost of trips by £32.80 on a single fare.
Which? said that a same-day peak fare from London to Swansea would cost £109.50. But if someone bought a single from London to Swindon and another from Swindon to Swansea it would reduce the cost of the overall journey to £76.70.
Bob Crow, general secretary of the RMT transport union, said: "Once again we see the train operators milking every last penny that they can out of their franchises - even down to the point of charging first-class fares for first-class services that don't even exist.
"The private train operators are repeat offenders when it comes to mugging passengers and it's all about jacking up their profits. The Government should intervene and should sack train operators caught in this routine exploitation of the travelling public."
A First Capital Connect 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."Reuse content