Buses are replacing trains between Southampton and Winchester.
Workers on South Western Railway are to strike for 27 days before the end of the year in the long-running dispute over guards on trains.
Network Rail and SWR have apologised to its customers following major delays on its services this morning.
Passengers, planning to travel on South Western Railway, are being advised not to travel on its service.
Rail, Maritime and Transport (RMT) Union has announced that workers on South Western Trains will take part in a five day walk-out.
South Western Railway says it plans to deliver more than half of its normal Saturday service.
Services at a stand on South Western Railway both ways between Fareham and St Denys due to fallen tree on the line.