Firefighters and police investigating cause of fire that led to Heathrow shutdown

LONDON (Reuters) -London firefighters are working with police to investigate the cause of a huge fire at an electrical sub-station which shut down Heathrow Airport on Friday, a fire-fighting chief told reporters.

Huge orange flames and plumes of black smoke shot into the sky around 11 p.m. (2300 GMT) on Thursday as a blaze engulfed the substation.

"The fire involved a transformer comprising 25,000 litres of cooling oil fully alight," Jonathan Smith deputy commissioner at the London Fire Brigade.

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

That forced Heathrow, Europe's busiest airport, to shut, disrupting flight schedules around the world and prompting speculation as to what had caused the blaze.

British energy minister Ed Miliband said earlier on Friday there was no suggestion that there was foul play involved.

Asked whether the police were investigating the cause of the fire as a possible terrorist incident, Smith said he was unable to comment.

"All I can say, is the Metropolitan Police are investigating the cause of this fire, ably assisted by our fire investigation officers," he said.

Smith said 10% of the original fire remained alight and his officers were working to safely resolve the incident, adding that they would work with National Grid as they assess the site and attempt to restore power.

(Reporting by Sarah Young; Editing by Kate Holton)