Buses will replace trains between Fareham and Eastleigh for at least the next two days following a landslide.
It happened on Friday afternoon following days of heavy rain.
Engineers worked through the night to repair the line but an inspection found more work was needed.
Trains are being diverted, extending journey times by up to 30 minutes.
Buses are running between Eastleigh and Fareham calling at Hedge End and Botley and First Great Western services are being diverted via Southampton Central.
A spokeswoman for South West Trains said buses will replace trains for at least two days.
Trains from Portsmouth to London Waterloo via Fareham will be affected.