Rail passengers will learn on Tuesday how much their fares will go up by in January next year, amid protests about the high cost of tickets.
Regulated rail fares will rise by 3.5% from January next year, following the release of July's inflation figures.
The rise is set by adding 1% to July's figure for Retail Prices Index (RPI) inflation, due to be announced later.
Under the formula, fares in England and Wales are due to go up by July's RPI measure of inflation, plus 1%.
It will take the total increase in fares to about 25% since the coalition came into power in 2010, says the Campaign for Better Transport (CBT).
RPI - the Retail Prices Index - was 2.5% last month, according to the Office for National Statistics (ONS).
Labour said rail fares in the UK were already among the highest in Europe.
Labour accused the government of 'ripping off' passengers, but the government defended the rise.
The rise includes season tickets. Most season tickets are "regulated" - that is, prices are set by the government.
Regulated fares have increased by more than the rate of inflation in most years since 2004.
Michael Roberts, director general of the Rail Delivery Group, said that although rail fares had increased above inflation for a decade, the number of passengers had seen "phenomenal growth".
Such fares include season tickets, "anytime" single tickets around major cities, and off-peak inter-city return tickets.
"Compared with fifteen years ago, we're now carrying twice as many passengers as we did then," he said, but added that rail services were partly a "victim of that success" because there was now overcrowding on some major services.
But under a "flex" rule, train companies can raise fares by 2% above the average, as long as the overall average stays at RPI plus 1%.
However, he said that the level of customer satisfaction with the railways had "steadily increased", and that an EU-wide customer survey had shown UK passengers were among the most satisfied in Europe.
As a result, some fares could increase by 4.5% next year.
Money raised from fare increases would be re-invested in rail infrastructure, a rail expert said.
'More to do'
Rail journalist Philip Haigh said that although the government raises more money through fare prices, it is then put back into the network.
Labour said it would abolish the flex rule, and accused David Cameron of failing to stand up for working people.
'Inflation-busting fares'
"He's allowed train companies to sting passengers with inflation-busting fare rises of over 20% since 2010, costing them hundreds of pounds," said Mary Creagh, the shadow transport minister.
If RPI stays at its June 2014 level of 2.6%, fares will increase by about 3.6%.
This year -2014- the government capped fare rises to keep them in line with inflation. But it is not known whether the same will happen in 2015.
But under a "flex" rule, train companies can raise fares by 2% above the average, as long as the overall average stays at RPI plus 1%, so some fares could increase by 5.6% next year.
The Department for Transport defended the rise.
The CBT said that since 2010, wages had risen by 6.9%, while fares had risen by 24%.
"We fully recognise there's more to do to bring down the cost of rail travel in Britain," said Transport Secretary Patrick Mcloughlin.
Last year the government capped regulated rail fares to rise in line with inflation.
"But we need to do it responsibly and we can't spend money we don't have," he said.
However, shadow minister for transport Mary Creagh said: "David Cameron has failed to stand up for working people struggling with the cost-of-living crisis.
The Scottish Government has already said rail fares in 2015 will be capped at RPI - in other words at 2.5%.
"He's allowed train companies to sting passengers with inflation-busting fare rises of over 20% since 2010, costing them hundreds of pounds."
'Priced off' trains
She said the Conservatives had "botched" the privatisation of the railways.
During a speech in London later, Ms Creagh will add: "Our rail fares are among the highest in Europe. Rail passengers rightly feel ripped off when they are uncertain if they paid the lowest fare."
She said Labour would impose a "strict cap" on rail fares, removing the "flex" arrangement.
Public transport campaigner at CBT Martin Abrams said the cost of taking the train to work had become a "huge part" of living costs.
He added: "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."
