Rail services up and down the country have been affected by a shortage of staff caused by rising cases of Covid-19.
A number of operators have blamed the Omicron variant for cancellations in the build-up to Christmas.
Many have brought in reduced timetables as a result of staff being either sick or isolating.
Passenger numbers have been hit by the advice to work from home, but are likely to increase this week as people make festive trips.
Latest preliminary figures from the Department for Transport show demand for trains was at 60 per cent of pre-pandemic levels on December 9.
CrossCountry said it was cancelling dozens of trains and cutting carriages on others and anticipated “widespread disruption”.
It told travellers: “Please pull your journey forward to sooner rather than later to get to your end destination as early as possible.”
It has made cancellations on routes including Manchester to Bournemouth via Birmingham; Bristol to Paignton, Devon; Cardiff to Nottingham; and Birmingham to Stansted Airport.
Meanwhile, 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.
“Where possible, we’ll contact customers in advance to let them know about cancellations but customers should also check before they travel.
“Tickets will be valid on trains before and after a cancelled service. To support customers and spread demand, we have removed all peak restrictions over the Christmas period.
The firm has removed all peak ticket restrictions over the Christmas period in a bid to spread demand and tickets for cancelled trains will be valid on services before and after a cancelled service.
Govia Thameslink Railway said a train crew shortage means there will be a reduced service across the Southern, Thameslink and Great Northern network until the end of Christmas Eve.
Greater Anglia said it has removed trains from its timetables due to falling passenger numbers “as people follow advice to work from home”.
It said: “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.”
Passengers who have booked a ticket on a cancelled service can travel on either the train before or after.
Northern said it is operating amended timetables due to “Covid and crew unavailability and major engineering works”. Disruption is expected to continue until at least January 3.
ScotRail is running an amended service on several routes until further notice due to Covid-19 related illness.
TransPennine Express said delays and short-notice cancellations over the next fortnight are 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.”