London Underground services suffered severe disruption on Monday morning due to “power supply problems”, Transport for London (TfL) said.
The entire Victoria line was suspended at about 7am, before later reopening.
The Piccadilly line was suspended between Hyde Park Corner and Cockfosters, with severe delays on the rest of the line.
Tube stations including Oxford Circus and King’s Cross were evacuated and closed due to the problem.
The disruption came as TfL warned that some of its services would be busier than normal with many people visiting London to pay their respects and attend ceremonial and commemorative events after the death of the Queen.
The Rail Delivery Group said journeys into London and at stations in the capital would be “extremely busy”, particularly on the day of the Queen’s funeral on Monday 19 September.
“Understandably, many people wish to pay their respects to Her Majesty the Queen during this period of national mourning,” said Jason Webb, the Rail Delivery Group’s customer information director.
“On the day of the funeral, Monday 19 September, people should plan carefully the timing of their journey home, as trains and stations are likely to be extremely busy.”