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, King's Cross and Euston and is used by more than 800,000 people every day.
The service had reopened by 13:00, but is now running with minor delays.
Commuter Mario Toubes-Rodrigo said he missed his pre-booked train to Milton Keynes because it took 75 minutes to make a journey that normally takes 20 minutes.
"I waste so much time and money on the Northern line," the microbiologist said.
"I get up to leave my house even earlier and pre-book my train tickets to avoid problems but I still end up having to buy on the day tickets and turning up late to work."
There is still no service on the Northern Line between Camden Town and Kennington via Charing Cross.
Tube tickets are being accepted on Southeastern and Thameslink trains, as well as on trams and buses.
The Metropolitan Line has minor delays as a result of the signals failure. Problems were also reported on the Bakerloo line, but have since cleared.
Replacement buses through London were earlier reported to be overcrowded, with one commuter complaining it was "everyone for themselves".
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.
People also said they had been forced to miss trains from King's Cross and Euston because of the disruption.
First day of term for us.— Lara Conner (@larabconner) September 4, 2019
Chucking it down ☔
Northern Line completely suspended
Official. I hate the northern line. I did not realise it was possible to hate non sentient objects. On the plus side I have just completed 30 min walk to nearest Thames link station. Thankfully Thames link trains are very reliable. trains........... pic.twitter.com/YQ5XTVhiiC— Paul Radomskij (@PaulRadomskij) September 4, 2019
Get up at the crack of dawn as I’m travelling to Manchester this morning. Try to get on the @northernline to kings cross and there are no trains...if I miss my train and my meetings will @TfL reimburse me??????????— LA (@Ele_ctrified) September 4, 2019
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."