Millions of rail users in the UK are bracing themselves for news of an increase in regulated rail fares from January 2018.
Train operators are allowed to raise fares by as much as the Retail Prices Index (RPI) figure for July, expected to be in the region of 3.5%.
The exact figure will be published later this morning.
Passenger groups said commuters would be worst-hit, and suggested that the RPI measure should be scrapped.
The rises will affect “anytime” and some off-peak fares as well as season tickets in England and Wales.
In Scotland, it is mainly commuters who will be affected, with off-peak fares rising by a smaller amount.
The Scottish government currently limits rises in off-peak fares to RPI minus 1%.
There are no plans for increases in Northern Ireland.
Unregulated fares, which include super off-peak travel and advance tickets, will be set in December.
Transport Focus, which represents the interests of passengers, said rail users were already fed up with getting poor value for money.
Analysis: Richard Westcott, transport correspondent
Oh the irony… Regulated fares were meant to be the government’s way of stopping private rail firms from overcharging passengers.
They apply to tickets where people don’t have much choice but to go by train: commuting into big cities, for example.
But for many years, ministers have deliberately used the system to put prices up anyway. Why? Because they want passengers to pay a bigger chunk of the rail bill, so that the government pays less.
Fares used to account for about half the cost of running our trains. Today it’s about 70%.
It does mean, of course, that people who don’t commute by train, which is most of the country, pay less to subsidise the system.
But that’s little consolation to workers who’ve faced consistent price rises that have often outpaced their salary. Even allowing for inflation, rail fares have gone up by about 25% since the mid-1990s.
I’ve spoken to many passengers – often young people at the start of their careers – who’re on the brink of changing jobs because they can’t afford the increases.
“Wages are not keeping pace with inflation and performance remains patchy,” said a spokesperson for the group.
“Passengers, especially commuters, face potential strike action, the consequences of the continual rise in passenger numbers, and disruption caused by railway upgrades.”
Transport Focus said it would also like to see the RPI measure replaced by the Consumer Prices Index (CPI), which is currently running at 2.6%.
CPI is typically lower than RPI.
However the government said the fare increase was justified by improvements to the network.
“We are investing in the biggest rail modernisation programme for over a century to improve services for passengers – providing faster and better trains with more seats,” a spokesperson for the Department for Transport said.
“We have always fairly balanced the cost of this investment between the taxpayer and the passenger.”
The Rail Delivery Group, which represents train operators, said there would be an extra 170,000 seats for commuters by the end of 2019.
The Department for Transport also rejected the idea of using CPI to determine price rises.
It said RPI was used across the rail industry – for example in calculating the cost of running train services.