A single "fuse" which was wrongly installed caused the power cuts that plunged parts of London and the South-East into darkness last month.

National Grid said an over-sensitive automatic protection relay disconnected supplies to three power stations on August 28, leaving about 410,000 homes and businesses without electricity.

In its report on the power failure published yesterday, the company said it was "the largest loss of supply from National Grid for more than ten years".

The power failure resulted from a combination of events which began when the national control centre received an alarm from Hurst power station in Kent at 6.11pm.

According to the report, the alarm indicated that a transformer was "in distress and could fail, potentially with significant safety and environmental impacts".

The transformer had to be shut down, leaving Hurst supplied only by the sub-station in Wimbledon, south-west London, via another in New Cross, south-east London.

But seconds later, the change in power flows was interpreted as a fault by automatic protection equipment.

This automatically disconnected New Cross, Hurst and part of Wimbledon from the rest of the transmission system, causing the loss of about 20 per cent of total London supplies.

Power was restored to all sub-stations within 37 minutes, the report said.

National Grid said it had informed Scotland Yard that the power failure was "a system incident with no third party involvement".

Following an investigation, National Grid found that scheduled maintenance work being carried out that day could not have caused the incident.