Flights have resumed at Heathrow Airport and a full service is expected on Saturday following an “unprecedented” loss of power caused by nearby a substation fire.

About 200,000 passengers were affected as flights were grounded throughout Friday, with inbound planes being diverted to other airports in Europe after flames ripped through the North Hyde plant in Hayes, west London, on Thursday evening.

The airport’s chief executive Thomas Woldbye apologised to stranded passengers and said the disruption was “as big as it gets for our airport” and that it could not guard itself “100%”.

The Met Police confirmed the fire was not believed to be suspicious.

The investigation will focus on the “electrical distribution equipment”, the force said.

British Airways announced eight of its long-haul flights had been cleared to leave Heathrow during Friday evening and it was “urgently contacting customers to let them know”.

Restrictions on overnight flights have also been temporarily lifted to help ease congestion, the Department of Transport said. Passengers have been advised to contact their airlines for the latest updates.

Mr Woldbye said that a back-up transformer had failed meaning systems had to be closed down in accordance with safety procedures so that power supplies could be restructured from two remaining substations to restore electricity enough to power the airport.

Several airlines announced they would restart scheduled flights both to and from Heathrow, including British Airways, Air Canada and United Airlines.

An airport spokesperson said the first flights were focused on “repatriating the passengers who were diverted to other airports in Europe… and relocating aircraft”.

Mr Woldbye said: “I’d like to stress that this has been an incident of major severity. It’s not a small fire.

“We have lost power equal to that of a mid-sized city and our backup systems have been working as they should but they are not sized to run the entire airport.”

Asked if there is a weak point in Heathrow’s power system, he said: “You can say that but of course contingencies of certain sizes we cannot guard ourselves against 100% and this is one of them.

“I mean, short of anybody getting hurt, this is as big as it gets for our airport.”

“This is unprecedented,” he added.

Mr Woldbye went on to say the airport expected to return to “100% operation” on Saturday.

Transport Secretary Heidi Alexander said she believed airport bosses “stood up their resilience plan swiftly, and they’ve collaborated closely with our emergency responders and the airline operators; they do have backup energy supplies, they have generators, diesel generators.

“None of that failed on this occasion because that backup supply is designed to protect the critical key systems within the airport and not to provide power to the whole airport.”

Alexander added that she was in close contact with the energy secretary, the home secretary and with Heathrow to “make sure that any lessons we need to learn from the systems that the airport has in place are learned”.

Ofgem, the energy regulator, earlier announced it would commission a review “to understand the cause of this incident and what lessons can be learned”.

Emergency services were first called to the scene at 23:20 GMT on Thursday.

Some 120 aircraft heading to the airport at that time were forced to either divert or return to their point of origin.

Ten fire engines and about 70 firefighters were sent to tackle the blaze, LFB said, with the fire being brought under control by 06:30.

London Fire Brigade (LFB) said the fire involved a transformer containing 25,000 litres (5,500 gallons) of cooling fluid that had been set alight.

A large cordon was put in place as a precaution and about 150 people were evacuated from their homes. Most of those had returned home by 17:00, according to LFB.

Hillingdon Council said it was assisting 12 people with hotel accommodation until it was safe for them to return to their homes.

More than 65,000 homes in the area were left without power as a result, as well as the airport, although the National Grid said power was restored by 14:00.

Source: BBC