Britain's biggest blackout for 25 years, which plunged large parts of London into darkness, was the result of a one amp fuse being fitted in place of a 5,000 amp fuse at a substation.

National Grid Transco, which runs the transmission network, yesterday came up with the explanation for the 37-minute blackout on August 28 following an investigation ordered by the government.

Roger Urwin, the chief executive, denied it was the kind of "demonstrable negligence" that would put the privatised utility in line for a £100m fine from the industry regulator.

He said the fuse - which costs £2,000 - was fitted by a contractor from a firm he refused to name. There is no intention to take action against Grid employees who were meant to check the work.

However, Mr Urwin admitted that his company could face legal action from around 100 major customers who rely on its services.

The fuse was a "shoebox-sized" protection switch at a substation in Wimbledon that shut down automatically but prematurely when a surge of current came through.

An alarm had already indicated a problem with a transformer at Hurst in Kent - leaving south London dependent on Wimbledon.

The shutdown there led to the loss of 20% of the capital's electricity affecting 410,000 homes and businesses at 6.20 in the evening, and paralysing tube and rail services.