Thousands of rail passengers are facing travel chaos after all services out of London Euston were suspended due to a signal failure.
National Rail said all lines had since reopened but severe disruption is expected with some cancellations and delays of up to an hour.
It has apologised and warned rail users to “check before they travel”.
Among those affected are passengers who planned to travel on Virgin Trains.
The train operator said customers could use their tickets on Friday or apply for a full refund.
London Northwestern, which runs services from London to north-west England, said customers could use their tickets on East Midlands Trains, Chiltern Railway, Cross Country, Great Western Railway, London Overground, Thameslink, Virgin and Southern Rail.
Commuters stranded at the busy rail hub have been tweeting about their frustration, with many posting pictures of the packed station.
A BBC sports reporter who got managed to get on an alternative train from King’s Cross was told by LNER that Virgin tickets would not be accepted.
Virgin has since tweeted Drew Savage to say “we still have ticket acceptance in place with @LNER between King’s Cross and Leeds.”
Meanwhile several said nearby Marylebone station was also “in chaos” as passengers sought to make alternative journeys.
Others tweeted about the lack of air conditioning on the few overcrowded trains making it out of Euston.
The station doors were closed to prevent overcrowding according to Mr Savage who added officials had been handing out bottles of water to commuters.
British Transport Police said officers were on hand “for reassurance” amid the disruption.