People travelling into London Waterloo station are facing severe delays caused by a signal failure.
The major signalling fault is affecting services on the South Western Main Line between Woking and Surbiton.
South Western Railway is advising passengers not to travel and seek alternative arrangements.
Network Rail said its engineers were already on the site but the problem was likely to cause disruption until about 12:00 BST.
Up to six lines are affected by the signalling issue, including Alton, Basingstoke, Woking, Portsmouth Harbour and Dorking services into London Waterloo.
Network Rail said the signal failure was affecting all lines between Woking and Surbiton, meaning trains are unable to run on this line through to London Waterloo.
Trains between Salisbury and London Waterloo will start and terminate at Basingstoke, it added.
South Western Railway tweeted that passengers travelling on its main line should not travel.
But frustrated passengers have been venting on social media about the company’s advice.