"Those challenges will be ratcheted up by the plans for night running, plans that the politicians say they now want to extend across the system and on to London Overground."
Nick Brown, London Underground's chief operating officer said: "I apologise to customers for the disruption they've suffered this morning on some Tube lines due to problems including signal failures and faulty trains.
"Our engineers are working as quickly as possible to restore services on the affected lines. We will be refunding customers who were affected by the disrupted services this morning."
