Rail operators across the South East have cut services from Monday (26/07) as they battle staff shortages.
It comes after "a significant rise" in workers needing to self-isolate, due to Covid-related absences in recent weeks.
A reduced weekday timetable is running on Thameslink and Southern services across five routes. It's hoped the changes will mean there will be fewer short notice cancellations but the operators say there could be further last minute changes at weekends.
Meanwhile, Great Western Railway has scaled back some of its 'Inter-City' services that link Reading to the West Country.
Passengers are being told to check the train times before travelling.
Which Thameslink routes are affected?
Between Sutton and St Albans City via Wimbledon and via Hackbridge;
Between Gatwick Airport and Bedford
Which Southern routes are affected?
Between Clapham Junction and Watford Junction/Milton Keynes;
Between Beckenham Junction and London Bridge via Dulwich;
Between London Bridge and Caterham/Tattenham Corner
Steve White, Chief Operating Officer for Govia Thameslink Railway which operates Thameslink and Southern, says the rail industry continues to be affected by the pandemic.
He says: "Unfortunately, like other industries across the country, coronavirus continues to affect our operations. We have fewer colleagues available at the moment due to a significant increase recently in the number of our people affected by Covid-19."
"Our colleagues have continued to work tirelessly throughout the pandemic and we're really sorry for any inconvenience caused by the latest changes."