A power cut caused "unprecedented" chaos on London's rail and Tube lines during the rush-hour last night.

Thousands of passengers were stuck in tunnels when trains ground to a halt at about 6.30pm due to a fault on the National Grid.

And about 270 sets of traffic lights were out in many areas, causing confusion on the roads before power was finally restored at 7pm.

London Mayor Ken Livingstone said at least 250,000 people were affected by the blackout.

The National Grid said it could take distributors time to restore supplies to customers and it was investigating the cause of the fault.

Spokesman Sean Regan said: "There was a fault in the 275,000 volt system affecting a ring around London, which occurred at 6.26pm.

"Power to the distribution network in London was restored at 7pm.

"It is going to take the regional distribution network some time to restore supplies.

"We are obviously investigating the cause of the fault."

Network Rail spokesman Kevin Groves said trains had been badly hit and the situation had been "unprecedented".

"All power to all our signal systems to the third rail, which supplies power to the trains, all our power to points and lighting, have all gone since 6.30pm.

"Trains have got no power at all. They are at a standstill.

"We have back-up generators but they can't be used in a power failure of this magnitude. You practically need a power station."

The power at Guy's and St Thomas's hospitals went off momentarily, but a back-up generator started before any damage was caused.

Mr Livingstone said: "It is totally unacceptable that this has happened.

"Under-investment in the National Grid must not be allowed to cause this kind of chaos in a city like London. This event demands the fullest inquiry to ensure it does not happen again.

"Investigations are under way into the cause of the power cut, but there is no evidence of it being caused by a terrorist attack."