Govia Thameslink Railway's head of network operations has described the company as "really sorry" for the impact of strikes this weekend.
Southern, Thameslink and Great Northern services are set to be affected on Saturday (July 29) and Sunday (July 30), as members of the RMT union undertake industrial action.
An amended timetable with fewer services will be in place on Saturday, whilst a late start to services is expected on Sunday.
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According to Govia Thameslink Railway, trains "are likely to be busy", customers may have to queue and might not be able to board their usual train.
As a result, passengers have been urged to check their journeys before setting off, on the National Rail website.
Chris Fowler, Govia Thameslink Railway's head of network operations, said: "We’re really sorry about the impact that this round of industrial action is having on our customers, especially as many will be looking to start their summer holidays.
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"We’re running as many trains as we can, but unfortunately on most routes there will be some times where people will be left without a service.
"It’s incredibly important that people check their journey before they travel and leave plenty of time, as trains will be much busier, particularly where services are starting later and finishing earlier than usual.”
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