London commuters were left stranded by the power cut

In its preliminary report published on Wednesday, Ofgem said the power cuts - which affected more than 600,000 homes and businesses - were not related to any shortfall in funding.

It said it was very concerned about the failures and its investigation would continue, focussing on whether the National Grid company's procedures were in line with its legal duty to maintain the network efficiently.

It also plans to investigate whether London Underground was given enough warning that maintenance was being carried out on the grid.

About 1,600 trains were stopped and 60% of the London Underground network was closed, stranding 250,000 people during the evening rush hour.

'Isolated incidents'

The power cuts caused travel chaos in London on 28 August, leaving about 410,000 homes and businesses left without electricity.

The following week saw another 220,000 customers affected by a similar power failure in east Birmingham, caused by a fault on the National Grid.

Both blackouts were under an hour in duration.

Ofgem's managing director of competition and trading arrangements, Boaz Moselle, said the power cuts were "isolated incidents".

"Total investment since privatisation in the electricity network is over �16 billion a level which is much higher than before privatisation.

"The National Grid network in this country is 99.9999% reliable and power cuts on the low-voltage distribution networks have fallen by 11% since 1991/92."

A full report from Ofgem was expected by the beginning of next year.

In its own report into the London outage, the National Grid said it was its largest power cut for 10 years.