Rail passenger watchdog Passenger Focus has criticised First Great Western for increasing fares, only three months after reducing them.
First Great Western cut some of its single fares by half in September to attract more customers, but now it has put some back up by as much as 15%.
A Plymouth to London off peak single ticket goes up from £43 to £49.50.
First Great Western, which operates in Devon and Cornwall, said the cuts had not had the effect it was looking for.
Train operators in the South West are also introducing new limits on when cheaper off-peak tickets can be used.
Anthony Smith, Passenger Focus chief executive, said: "Train companies are effectively forcing passengers into buying more expensive tickets."