Train companies are under no obligation to offer passengers the cheapest tickets available, one of the largest firms has claimed. According to emails that came to light on Friday, it is First Great Western’s policy to presume that people have already researched the cheapest fares “by the time the passenger is requesting their tickets at the booking office”.

In the emails, the company said it was “unrealistic” to expect its staff to ensure that the tickets being sold were the cheapest available. That meant some passengers could be sold the higher-priced “anytime” tickets during off-peak hours.

The policy, described in emails obtained by the Daily Telegraph, was criticised by the rail minister Claire Perry. She said: “The most important part of the railway is its customers, and they must be able to trust that they are getting the best possible deal every time they travel. If this is not happening it is unacceptable.

“Train companies must get better at giving their customers information about ticket options in a straightforward way. I will not hesitate to hold operators to account if they do not take this issue seriously. I have been clear that the industry must improve and simplify the system so that customers can be confident of making the right choice when buying tickets.”

In a statement that appeared to reference a customer complaint about the matter, a First Great Western spokesman said: “Our ticket office staff should always do their best to find out as much about a customer’s journey as possible so they can sell the most appropriate ticket for their needs.

“Our response should have reflected this, and it looks very likely that this customer is due a full apology and a refund. We have reopened this customer’s complaint and will be investigating further.”

A spokesman for the Rail Delivery Group, which represents train operators and Network Rail, told the Telegraph: “If someone doesn’t know the type of ticket they want, staff are expected to ask what the passenger’s travel needs are so that they can be sold the most appropriate ticket for their journey.”