Rail operators claim the covid-19 pandemic has led to staff shortages and caused widespread train cancellations in the run-up to Christmas.
It comes after a number of railway firms have begun running reduced timetables as many workers are either sick or isolating.
Rail provider CrossCountry announced it is "expecting widespread disruption to our services this week".
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Travellers are being urged to move their journeys forward to avoid disruption.
The company has been forced to cancel dozens of trains each day while many others have fewer carriages than usual.
Among the routes affected by the cancellations include Manchester to Bournemouth via Birmingham; Bristol to Paignton, Devon; Cardiff to Nottingham; and Birmingham to Stansted Airport.
Another train firm, Avanti West Coast, told passengers: “The pandemic is, unfortunately, resulting in some staff shortages.
“We’re doing everything we can to run our full timetable but there may be some short-notice cancellations.”
In an attempt to increase demand, all peak ticket restrictions over the Christmas period have been removed by the firm.
Train crew shortages have also affected Govia Thameslink Railway who announce there will be a reduced service across the Southern, Thameslink and Great Northern network until the end of Christmas Eve.
It pointed to falling passenger numbers as one of the main reasons it removed trains from its timetables "as people follow advice to work from home."
It added: “We also have to plan for our staff being affected by the Omicron variant, especially as we’re already starting to see the early signs of its impact, to ensure we can continue to provide a reliable service.”
If passengers have booked a ticket on a cancelled service, they're allowed to travel on either the train before or after with that same ticket.
Northern also said it is operating amended timetables due to "Covid and crew unavailability and major engineering works".
Disruptions are expected to continue until January 3 at the very least.
ScotRail is another train firm running an amended service due to Covid-19 related illnesses.
TransPennine Express has also acknowledged delays and cancellations over the next fortnight due to "a lack of available staff caused by industrial relations issues".
A spokeswoman for the Rail Delivery Group, which represents train operators, said: “Like everyone else, our staff are susceptible to the virus but as we showed last year during the pandemic, we will ensure that key workers can get to where they need to be.
“We aren’t able to run every train as planned at the moment but we know people want confidence that their train is going to turn up so we will be working hard to give clear, accurate and timely information and people should check before they travel.”
Anthony Smith, chief executive of independent watchdog Transport Focus, said: “It’s better for some services to be temporarily withdrawn on a planned basis than to have chaotic last-minute cancellations.
“These are harder for passengers to deal with and more likely to lead to overcrowding.
“Passengers need clear, reliable information to plan their journeys. Anyone who has already bought tickets must be told and offered alternatives or their money back.”
The latest preliminary figures from the Department for Transport show demand for trains was at 60 per cent of pre-pandemic levels on December 9.
New advice to work from home has been pointed as a major reason for declining passenger number - but it's expected to increase this week as people travel for the Christmas holidays.
