Commuters are facing rush-hour chaos this morning after four Tube lines were hit with delays,.
The Circle line was suspended this morning due to earlier faulty trains. It is now running with severe delays.
The same issue is affecting the Disrict line, which is part suspended between High Street Kensington and Edgware Road.
It has minor delays on the rest of the line.
The Hammersmith and City lines is also part suspended between Baker Street and Aldgate.
It is running with severe delays on the rest of the line, with passengers advised to use their tickets on London buses.
The Hammersmith & City line also has severe delays due to faulty trains.
Dozens of people have taken to Twitter to complain of the delays they are facing on their way into work.
Nick Dent, Director of Line Operations at London Underground, said: "We are sorry for the problems with the service on the Circle, Hammersmith and City, Metropolitan and District lines this morning and will be refunding all customers affected.
"We are investing £5.4 billion in modernising the trains, track and signalling on these lines, in one of the most important upgrades in the history of the Tube network.
"The introduction of new signalling and removal of older infrastructure will result in greater capacity and improved frequency, as well as better journey times, reliability and customer information.
"We are working around the clock to minimise the impact of the introduction of the new signalling system, and appreciate the continued patience of our customers while we work to deliver long-term improvements to their service."
The delays come as rail passengers face travel misery throughout December due to series of strikes have started in a long-running dispute over guards on trains.
December Rail strikes
Members of the Rail, Maritime and Transport (RMT) union on South Western Railway (SWR) will stage walk-outs over 27 days, after peace talks with the rail provider collapsed last month.
