The 14.5 per cent price hike will affect travellers who use SuperSaver tickets - which can be used on any day except Fridays - on Virgin CrossCountry, whose sprawling rail network visits all of Britain's major towns, except London.
Rail campaigners attacked the move. Jonathan Bray, of the Save Our Railways pressure group, said: "Virgin is systematically putting the cost of turn-up-and-go travel through the roof." John Moorhouse, secretary of the Rail Users' Consultative Committee, added: "We are not pleased with the company pricing up. It means more expensive travel for a lot of people who turn up and just want to catch the next train."
The Government is considering measures to restrict the ability of private train operators to introduce higher fares - which ministers say will price people off the rail network.
Not only will passengers face more expensive tickets, but the CrossCountry service remains notoriously irregular - with more than one in seven trains "officially late".
"The service is appalling already. I cannot believe that by raising ticket prices you will attract more people," said Helen Jones, Labour MP for Warrington North, and a regular user of Virgin Trains.
Mr Branson's railway managers disagree. Virgin's policy is centred on its belief that private train companies will only become a success if they follow an "airline-style" booking system. This means passengers can only get cheap deals if they book ahead - by specifying their outbound and return journeys.
The company - which is expected to get approval this week to be floated on the Stock Exchange later this year - has even cut some the price of some pre-booked journeys. Travellers who buy "apex" tickets from Birmingham to Bristol pay pounds 21.20 for a return, but in the summer this drops to pounds 19.50.
Brian Barrett, Virgin Trains' chief executive, said yesterday: "This was an experiment we tried out last year and it has worked. Nearly 20 per cent of passengers now book ahead."
Last year, the company increased its cheap walk-on fares from London to Scotland by 15 per cent. Virgin expects that "40 per cent" of its revenue will be generated by book-ahead tickets in about two years' time.
According to an analysis of the company's fares by transport consultant Barry Doe, the price of an advance ticket has only risen by 2 per cent since 1995 whereas SuperSavers - the cheapest walk-on tickets - have rocketed by 22 per cent.
Despite the book-ahead fares, Mr Doe said that Virgin was putting up its ticket prices faster than any other operator. "Great Western Trains has frozen its pre-booked fares, why couldn't have Virgin?"
Premium price for a ticket to ride
Today's price From %
Birmingham - Bristol pounds 23.30 pounds 26.50 13.7
Swansea - York pounds 61.50 pounds 70.50 14.7
Plymouth - Leeds pounds 74.30 pounds 85 14.4
Liverpool - Winchester pounds 51 pounds 58.50 14.7
Manchester - Torquay pounds 58.30 pounds 67 14.9
Annual inflation 3.3 per centReuse content