Train companies base their annual fare increases for regulated fares (which include annual season tickets) on the retail price index (RPI) for July plus 1%.

Next month's RPI is likely to be around 4.5% which will mean thousands of rail passengers facing 5.5% annual increases in January 2009 - adding hundreds of pounds on to the price of an annual season ticket.

Although no official figures have been decided, it is thought that some fares could rise even higher.

Shadow transport secretary Theresa Villiers told the London Evening Standard: "All the signs are that the July inflation figures will mean more rail fare misery for commuters."

Gerry Doherty, general secretary of transport union TSSA, told the paper: "It is a scandal that the rail companies have this trigger which allows them to put up fares every year."

Around 40% of fares are regulated and fall into the RPI plus 1% regime, with train companies limited to how much they can raise prices annually.