Trains to Portsmouth are being diverted along some routes today after flooding in the Botley area caused a landslip along the railway embankment.
South West Trains has been left unable to run trains between Eastleigh and Fareham on the route via Botley and Hedge End.
The firm says engineers have assessed the site and says significantly more work is required than previously thought.
Further assessments will be made over the next few days and a date has not been set for the reopening of the line.
In the meantime, trains from London Waterloo to Portsmouth via Eastleigh are being diverted and will not call at Botley or Hedge End in either direction, extending journey times by up to 30 minutes.
London-bound services will not call at Farnborough Main, Woking or Clapham Junction to minimise the impact of the diversion delay on the rest of the train service.
Portsmouth bound trains will call additionally at Southampton Airport Parkway.