A spokeswoman for One, which runs Liverpool Street Station, said: "Network Rail's engineering works in the vicinity of Liverpool Street Station have overrun and it is not clear how long the station will be shut for. It is causing major disruption today. Our advice is not to travel unless you have to." She said she was unaware of what the problem was, but added: "Network Rail is working on overhead line equipment. Late last night it looked like it would finish on time, so it is something that has happened very recently. It is very important work they are doing which will enable us to run a better service when it is completed."