Passengers are facing travel disruption after an entire London Underground line was suspended due to a signal systems failure at the main control centre.
One commuter tweeted that he “hated the Northern Line”, which was suspended just before 06:00 BST.
The Tube line connects transport hubs Waterloo, Kings Cross and Euston and is used by more than 800,000 people every day.
Transport for London said it was “working hard to restore the service”.
Tube tickets are being accepted on Southeastern and Thameslink trains, as well as on trams and buses.
Both the Bakerloo and Metropolitan lines also have minor delays as a result of the signals failure.
The delays come as schools open for the new term this week and commuters on social media complained of having to queue to access stations.
Replacement buses through London are reported to be overcrowded, with one commuter complaining it was “everyone for themselves”.
People also said they had been forced to miss trains from Kings Cross and Euston because of the disruption.
Brian Woodhead, London Underground’s director of customer service, said: “I am extremely sorry for the disruption suffered by customers on the Northern line today following a signalling system failure at our control centre.
“Our engineers are working hard to fix the problem and restore a full service as quickly as possible.”