A survey of private train operators by the Opposition transport spokeswoman, Glenda Jackson, discloses that the companies plan to review fare levels every four months.
In the past, fares rose only once a year, in January. But they could now go up then, and again in May and September. Provision was made for interim increases when British Rail was restructured into separate business centres, though it has until now only been used for minor amendments for a small number of fares.
Labour said a survey of potential private rail operators showed that every one of them was intending to use the new regulations to review and amend fare levels every four months.
Spokesmen for the first three private rail franchises - InterCity Great Western, South West Trains and LTS Rail - all said they review rail fares three times a year, while InterCity East Coast said they intend to review fares "continuously ... any sensible train operator would do that to take account of demand".
Central Railways said it was "conceivable" that fares on their routes would rise in May and again in September. Cross Country Railways said it would look at fares at the same intervals, though it did not know what its plans were at this stage.
Ms Jackson commented: "It is clear that once franchises enter the private sector, regulations designed to allow minor changes in fare levels will be used to increase fares comprehensively not just once a year.
"Ministers have broken their promise to commuters that fares would not rise by more than the rate of inflation. Now they are set to hit commuters again, with what is effectively a privatised fare 'triple whammy'."
The prospect of further increases follows an announcement in December that fares would be rising by as much as eight per cent on some lines, despite ministerial assurances that fares would be capped at the rate of inflation. New fare levels, to be introduced in May, are due to be finalised on 4 February.