Trains passengers have been warned to expect delays this morning because of a signalling problem on the Portsmouth to London line.
South West Trains said the problem had developed between Clapham Junction and Vauxhall on the fast London-bound line.
It said a signal was not displaying properly, meaning trains have had to come to a stop before the signal and phone the signaller before continuing.
The company said trains between Portsmouth Harbour and London Waterloo may be terminated at Guildford or Woking, and trains will be then be available on the slow train line the rest of the way.
Some services may also be cancelled.
The disruption is expected to go on until at least 2.30pm while engineers try to fix the problem.
For further updates go to journeycheck.southwesttrains.co.uk.