LONDON – Flights to and from London’s Heathrow Airport were canceled Friday after a fire at a nearby substation knocked out power to Europe’s busiest airport, disrupting travel plans for hundreds of thousands of people around the world.

Here’s a look at what’s happening and its impact on air travel.

What happened?

A fire at an electrical substation in west London, about 2 miles (3 kilometers) from the airport, knocked out power to Heathrow Airport just before midnight on Thursday.

The “significant power outage” forced officials to shut the airport until 11:59 p.m. on Friday “to maintain the safety of our passengers and colleagues.”

The London Fire Brigade said 10 fire engines and 70 firefighters responded to a fire at the substation that was reported at 11:23 p.m. on Thursday. The fire has been contained but firefighters will remain at the scene throughout Friday, the fire brigade said.

Energy Secretary Ed Miliband said the “catastrophic” fire appeared to have knocked out a backup generator as well as the electrical substation that supplies power to Heathrow Airport.

National Grid, which maintains energy infrastructure in Britain, said the fire damaged equipment at the substation and crews are working to restore power supplies as quickly as possible. Power had been restored to 62,000 customers by 6 a.m. local time, but 4,900 were still without electricity.

What caused the fire?

The cause is still under investigation, but officials said there was “no suggestion” of foul play.

“We don’t know the cause of this fire. It’s obviously an unprecedented event,” Miliband said.

He said the fire and subsequent shutdown of Heathrow raises questions about the resilience of a key piece of national infrastructure.

How was Heathrow affected?

The disruption disrupted travel plans of the roughly 200,000 people who were expected to travel through Heathrow on Friday. Heathrow advised passengers not to travel to the airport and to contact their airlines to rebook flights.

With all take offs and landings canceled, the first impact was on dozens of long-haul flights from North America and Asia that were in the air when the airport was shutdown. Some were forced to turn around, while others were diverted to airports around the U.K. and Europe.

Heathrow-bound aircraft have landed at Schiphol Airport in Amsterdam; Shannon Airport in western Ireland; Glasgow, Scotland; Manchester, England; Charles de Gaulle in Paris; Lyon, France; and Frankfurt, Germany, among others.

The impact on short-haul flights was delayed until Friday morning because flight operations at Heathrow are severely limited between 11 p.m. and 6 a.m. every day to minimize overnight noise in surrounding communities. Even so, thousands of people will be unable to travel to and from airports around Europe and the U.K. on Friday.

Some 4,000 tons of cargo have also been stranded by the closure, according to Anita Mendiratta, an aviation consultant.

How long will the disruptions last?

Even if the airport reopens on Saturday, the disruptions are expected to last for days as airlines move stranded aircraft and flight crews back into position and work to accommodate passengers whose flights were canceled.

Mendiratta estimated that it would take two to four days to clear all the backlogs.

“This is an extreme situation where the entire aviation ecosystem is impacted,” Mendiratta said.

“There will be two things that will be happening as a priority number one shall we say. First is airport operations and understanding, from an electrical system point of view, what has been impacted, if anything,” she added. “Did anything short out, for instance? What needs to be reactivated? And then how do you literally turn the airport back on again? Passenger and cargo.

“On top of that, there’s the issue of actually managing the human component of it. You have passengers that are impacted, crew are impacted and operations — so being able to re-mobilize everything.”

How big is Heathrow?

Heathrow was Europe’s busiest airport last year, with 83.6 million passengers traveling through the airport. Its closure will have far-reaching impacts because it is a major hub for connecting flights to cities throughout Britain and around the world, as well as for travel to London.

Does London have other airports?

Yes. Five other air hubs in southeastern England identify themselves as London airports, but they are much smaller than Heathrow. Gatwick, Britain’s second-biggest airport, handled 43.2 million passengers last year. It is in the town of Crawley, 28 miles south of London.