HEATHROW and National Grid bosses face serious questions over how a single fire led to the closure of the airport and left tens of thousands of homes without power, Downing Street said today.

More than 200,000 passengers had their flights to or from Britain’s biggest airport cancelled or diverted after a transformer within the west London electrical substation caught fire.

“There are questions to answer,” a No 10 spokesman said. “We expect those questions to be answered but our clarity right now is on this incident being appropriately dealt with.”

Some 150 people were evacuated from surrounding properties after the London Fire Brigade was called to reports of a “huge bang” at 11.23pm on Thursday.

Deputy commissioner Jonathan Smith said: “The fire involved a transformer, comprising 25,000 litres of cooling oil, fully alight.

“This created a major hazard due to the still live high-voltage equipment and the nature of the oil-fuelled fire.”

Scotland Yard said that its Counter Terrorism Command would lead inquiries given the impact of the fire on critical national infrastructure, but there is currently no indication of foul play.

Heathrow was accused of a “clear planning failure” by the International Air Transport Association (Iata) as passengers were warned to expect disruption for several days.

”This is yet another case of Heathrow letting down both travellers and airlines,” said Iata director-general Willie Walsh, former CEO of British Airways. “And that begs some serious questions.”

“Firstly, how is it that critical infrastructure — of national and global importance — is totally dependent on a single power source without an alternative?”

Heathrow said that its back-up energy systems worked “as expected” when the substation fire started, but added the systems are not designed to run a full operation.

A National Grid spokesperson said: “We’re sorry for the disruption caused by a fire at our North Hyde substation and for the loss of power supplies in the area.

“Working in partnership with the local network operator SSEN Distribution, the network has been reconfigured to restore all customers impacted, including the ability to resupply the parts of Heathrow airport that are connected to North Hyde.

“This is an interim solution while we carry out further work at North Hyde to return the substation and our network to normal operation.”

Prospect union said its Air Traffic Control and energy sector members were managing “significant extra work” round the clock.