Labour and transport campaigners will step up their challenge to the railway system as the full extent of planned fare rises for 2015 is revealed on Tuesday, with some fares expected to rise by as much as 6%.

Campaigners have demanded action to tackle above-inflation fare increases, pointing out that ticket prices are rising nearly four times faster than wages and average fares will have risen 25% under the coalition government.

Regulated train fares are pegged to July's inflation figure and capped at RPI+1% – although train operators may raise fares on some routes by an extra 2%. The flex rule means some fares could rise by 5.6% if RPI follows economists' predictions – and could add more than £300 to the cost of annual season tickets on the most expensive commuter routes. The average rise will be 3-4%.

Cumulatively, prices will have gone up almost 25% during the current parliament, while average wages have risen by 6.9%. According to the Campaign for Better Transport commuters are spending up to a fifth of their income on season tickets.

Commuters in Greater Manchester and parts of Yorkshire will also face further expense from next month when Northern Rail introduces evening peak periods when it will charge the highest fares.

Martin Abrams, of the Campaign for Better Transport, said: "With people's wages stagnating, and in some cases falling, the expense of taking the train to work has become a huge part of living costs. If the government doesn't put an end to above-inflation fare increases quickly, ordinary commuters will be priced off the train and could be forced into agonising decisions such as moving house or quitting their jobs." Labour will seize on the fare rise to highlight what it terms "the choice" voters have on railways. Although the opposition has not pledged a freeze, Mary Creagh, the shadow transport secretary, will use a speech to engineers in London to warn that prices could rise by a further 24% by 2018 under Tory plans.

Creagh said: "David Cameron has failed to stand up for working people struggling with the cost-of-living crisis. He's allowed train companies to sting passengers with inflation-busting fare rises of over 20%.

"We can't go on like this. The choice facing passengers is between fares rising another 24% by 2018 under the Tories, or a Labour government which will cap annual fares on every route and enact the biggest railway reforms since the Tories' botched privatisation, delivering a better deal for passengers and taxpayers."

Labour said it would abolish the flex – although much of the sting for commuters was taken out last year when George Osborne cut the train operators' flexibility to an additional 2% rather than 5%. The chancellor also limited overall rises for 2014 to RPI rather than RPI+1%. Fares rose above inflation under the flex rule on two of the 20 busiest commuter routes.

Labour has also pledged to create a legal right for passengers to be sold the cheapest available ticket for their journey, after rail regulators' warnings that passengers are often confused and pay over the odds.

Shadow transport secretary Mary Creagh will criticise rate rises in a speech to engineers. Photograph: Ray Tang/Rex

Creagh will also promise to "ease the pressure on farepayers by passing on the savings from our reforms". While stopping short of unions' calls for renationalisation, Labour is to allow public sector companies to bid to run rail lines, and will create a rail authority, a "single guiding mind to plan investment and services", bringing Network Rail together with a representative passenger rail organisation. The new body would contract routes, coordinate services and oversee stations, fares and ticketing.

The transport secretary, Patrick McLoughlin, accused Labour of trying to rewrite its record, and said abolishing the flex would cost £100m.

He said: "The last Labour government oversaw year after year of inflation-busting fare rises – a mammoth 11% in their last full year.

"We fully recognise there's more to do to bring down the cost of rail travel in Britain. But we need to do it responsibly and we can't spend money we don't have. What Labour are proposing today is an uncosted spending commitment that would mean over £100m more government borrowing."

Rail unions and the TUC's action for rail campaign will mark Tuesday's inflation figures with protests at more than 40 train stations across the country. The TUC general secretary, Frances O'Grady, said: "It's grim news for commuters that they face yet another year of fare hikes above inflation, while their wages keep dragging behind inflation. The cost to passengers of the failed privatisation of our railways keeps growing year on year. We've ended up with slower trains and higher fares than countries who have kept their trains in public hands."

Mick Whelan, general secretary of Aslef, the train drivers' union, said privatisation had left "a fragmented system which is all about making a private profit at public expense". The rail industry's body, the Rail Delivery Group, withheld comment.

A recent Passenger Focus survey showed only 45% of passengers believed their train service provides value for money.