Rail passengers who book tickets well in advance may end up paying four times as much as those who wait until closer to the date, according to an investigation that exposes how operators have adopted tactics used by budget airlines to raise profits.

Britain’s five intercity train companies each claim that tickets are cheaper for passengers the earlier they book, but Virgin and Great North Eastern Railway (GNER) have been found to raise and lower prices repeatedly. This makes it impossible for passengers to be sure that they have obtained the cheapest fare.

The investigation by Which?, the consumer watchdog, found that a seat on a Virgin train from London to Manchester cost £12.50 one way when booked 12 weeks in advance. When researchers checked five weeks later, the price of a seat on the same train had risen to £60. Two weeks after that, five weeks from the travel date, the price had dropped to £12.50. With one week to go, the price was £20.

GNER’s fares fluctuated apparently at random, rising and falling between £20 and £40 for a one-way ticket from London to Newcastle upon Tyne.

A spokesman for Which? said: “We think that it’s confusing for passengers and does little to inspire confidence in the companies’ own advice.”

A spokesman for Virgin said: “It’s a commercial world. You often get these variations with airlines. If we have still got seats available because anticipated demand hasn’t materialised, then yes, the price can be altered downwards.” He said that Virgin offered 350,000 “Value Advance” discounted tickets each week but an average of 70,000 went unsold.

GNER said that it sometimes lowered prices closer to the travel date because of “unforeseen circumstances”, such as a large group of passengers cancelling their tickets.

Which? said that passengers were confused by the 70 types of train fare available and the 760 conditions of use.

Anthony Smith, chief executive of the rail customer watchdog Passenger Focus, said: “Passengers tell us that they find the rail fares structure complex and confusing.

“It is too often the case that passengers need to know the right questions to ask in order to get the best deal, yet you can get a bargain if you know how to hunt around. We have long been pressurising the industry to simplify the fares structure and make it simpler for passengers to buy tickets.”

Which? also found that poor advice about fares meant that some rail passengers were paying excessive prices for their tickets. Of 25 questions about walk-on fares put to station staff and the National Rail Enquiries (NRES) helpline, about half were answered correctly.

One ticket price given – for a single journey from London to Grantham in Lincolnshire – was £44.50 when the passenger could have travelled for £20 on a train leaving ten minutes earlier.

The price of another journey – from Southampton to Bristol and then on to Birmingham later in the day – was quoted as £91 when it should have been £48.

Some of the most costly misinformation was given for journeys where season tickets should have been recommended. Passengers making a return journey between Swindon and Penzance twice in the same week could buy a Freedom of the South West ticket for £70. But Which? said that both NRES and station staff quoted £67 per journey, a total of £134.

The NRES website proved to be a much more reliable source of information, answering all questions put to it correctly.

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