Train commuters will "refuse to pay" if season ticket prices continue to be hiked, a rail campaign group has claimed.

Railfuture issued the warning on the day passengers find out how much more expensive regulated fares could be from January 2.

The UK, Scottish and Welsh Governments set a cap on the annual rise in most of these fares - which include season tickets - linked to July's Retail Prices Index (RPI) measure of inflation.

The figure will be announced by the Office for National Statistics at 9.30am and is expected to be around 2.8%.

Over the past three years the number of journeys using season tickets has dropped from 712 million in 2015/16 to 625 million in 2018/19.

Bruce Williamson, spokesman for campaign group Railfuture, said passengers would "groan and whinge" when fares go up again.

He told the PA news agency: "It might be that we've now reached the point where we cannot simply put fares up and expect passengers to take the hit.

"They will just give up and refuse to pay. They will either find either another job or another form of transport."

Rail Minister Chris Heaton-Harris said: "It's tempting to say fares should never rise, but the truth is that if we stop investing in our railway then we'll never see it improved."

The Department for Transport confirmed that a railcard extending child fares to 16 and 17-year-olds in England and Wales will go on sale for £30 next week.

A 2.8% rise in season ticket prices would lead to an increase of more than £100 in the annual cost of getting to work for many commuters.

Examples of potential season ticket increases include:

- Brighton to London: Increase of £125 to £4,581

- Gloucester to Birmingham: Increase of £119 to £4,357

- Barrow-in-Furness to Preston: Increase of £117 to £4,285

- Edinburgh to Glasgow: Increase of £114 to £4,198

Rail regulator the Office of Rail and Road said regulated fares went up by an average of 2.8% in January 2019, following the July 2018 RPI figure of 3.2%.

Analysis by the Trades Union Congress (TUC) found that rail fares have increased at twice the speed of wages since 2009.

TUC general secretary Frances O'Grady said "another hefty fare increase" is the last thing passengers needs.

Industry body the Rail Delivery Group said 98p in every £1 spent in fares goes back into running the railway.

The organisation's director of nations and regions Robert Nisbet said: "No one wants to pay more to get to work but by holding rises down to no more than inflation, Government is ensuring that money from fares continues to cover almost all of the day-to-day costs of running rail services.

"This means private sector and taxpayer money can go towards improving services for the long term."

Bank of England governor Mark Carney has previously described RPI as having "no merit", adding that "virtually everyone" recognises the lower Consumer Prices Index (CPI) measure of inflation.

The Campaign for Better Transport (CBT) called for CPI to be used to set the cap in regulated fare increases.

CBT chief executive Darren Shirley said: "There is still no end in sight to these exorbitant increases that will cost commuters dearly from January. The Government should commit now to January's fares rise being linked to CPI."

Reforming the fares system so that it offers good value for passengers while keeping costs down for taxpayers is one of the aims of the Government-commissioned Rail Review.

The review's findings and recommendations are due to be published in a white paper in the autumn.

The Rail, Maritime and Transport union is holding protests outside more than 30 stations across Britain on Wednesday urging Transport Secretary Grant Shapps to renationalise the railways.

Mick Cash, general secretary of the union, said passengers have "had enough of being ripped off by the private rail companies' profiteering".