Train services across England are being cut due to staff self-isolating.
Reduced timetables will be introduced in an attempt to improve reliability after a spate of last-minute cancellations.
More than 600,000 people in England and Wales were instructed to self-isolate by the coronavirus app in the week to July 14.
Thameslink and Southern will cut its weekday timetables on five routes from Monday “until further notice”.
It warned that further changes could be required, including at weekends.
Steve White, chief operating officer at parent company Govia Thameslink Railway, said: “Regrettably, we have had to make the difficult decision to reduce some weekday services.
“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.
“By bringing in a reduced timetable now, it will help to reduce short-notice cancellations and give customers more certainty.
“We strongly advise passengers to check before they travel, including at weekends.”
Avanti West Coast is also cutting trains from Monday on its routes between London Euston and Manchester, Birmingham and North Wales.
A spokesman said this is to “manage staff shortages and ensure a reliable service”.
London Northwestern Railway will introduce a revised timetable with fewer services from Saturday.
The firm’s customer experience director Lawrence Bowman said: “No-one wants to see trains being cancelled, particularly at short notice.
“But we are experiencing more of this, across all our services, as increasing numbers of staff are getting pinged by the Test and Trace app.
“We are sorry that we have had to make the decision to alter some services.
“However, the changes will help us run a more reliable and consistent timetable for passengers across our network.”
Northern said it is “expecting disruption” this weekend due to “more people being asked to self-isolate”.
Routes in and out of Sheffield are likely to be the worst hit.
The operator said: “Our teams work really hard to find cover for services, however we find it more difficult to find cover for isolating colleagues over the weekends.”