Mark Menzies claims Virgin Trains is "stinging" passengers between Preston and London by charging more than those leaving Lancaster on the same trains.
Preston tickets cost up to £145 for a standard single in peak hours, while the longer journey can cost £42.50.
Virgin said the costs reflect the high number of Preston business travellers.
The company said for "many years" it had "relaxed" pricing structures on some of its longer distance routes, which serve more "leisure travellers".
But Fylde MP Mr Menzies claimed the company was "exploiting" business travellers "simply because they can get away with it".
Preston is about 230 miles from London Euston while Lancaster is about 25 miles further away on the same line.
To book a single journey next Tuesday from Preston arriving in London at 10:13, a ticket costs £145. The same journey from Lancaster costs just £42.50.
Mr Menzies said the prices had led to a "bizarre" incident in which one of his constituents was threatened with arrest after buying a cheaper ticket from Lancaster and boarding the train at Preston.
Mr Menzies said another passenger claimed he bought a ticket from London to Lancaster and, when he tried to get off early at Preston, he was told he must continue his journey through to Lancaster and then get a train back to Preston.
"It's the definition of madness," said Mr Menzies.
The MP, who is a member of the Transport Select Committee and says he will raise the issue in Parliament, continued: "This idea that people travelling to work should pay more is ridiculous. No-one can explain to me the rationale."
A Virgin Trains spokesman said: "Virgin Trains has for many years relaxed peak restrictions on some of its longer distance routes where we have more leisure travellers who are less likely to be commuting into London in the busy peak period.
"That's not the case at Preston where there are a lot of business travellers who need to travel at peak times.
"Of course, if passengers are able to be flexible and travel outside peak times from Preston they can get lower fares."
An MP has vowed to fight a rail firm's "barmy" price structure which has seen people charged more than three times as much to travel a shorter route.