Heathrow Airport said it was "fully operational" on Saturday but delays and cancellations were expected as Europe's busiest air hub resumed operations after a power station fire caused travel mayhem.

The electricity substation blaze knocked out power at the London airport, closing it for most of Friday before some flights began to arrive and take off later in the evening.

The shutdown affected thousands of passengers around the world and raised serious questions about the reliability of one of the UK's most critical pieces of infrastructure.

"We can confirm that Heathrow is open and fully operational today," an airport spokesperson said on Saturday morning.

"Teams across the airport continue to do everything they can to support passengers impacted by yesterday's outage at an off-airport power substation."

About 1,350 flights were affected by Friday's closure, according to the Flightradar24 tracking website. Around 120 Heathrow-bound planes were in the air when the closure was announced and had to be diverted.

Delays and cancellations were likely on Saturday as airlines scrambled to clear a backlog of flights.

"We have hundreds of additional colleagues on hand in our terminals and we have added flights to today's schedule to facilitate an extra 10,000 passengers travelling through the airport," the spokesperson said.

"Passengers travelling today should check with their airline for the latest information regarding their flight."

British Airways said it expected to operate around 85 percent of its scheduled flights at the airport throughout the day. The airline would normally run about 600 arrivals and departures on a Saturday.

"We are planning to operate as many flights as possible to and from Heathrow on Saturday, but to recover an operation of our size after such a significant incident is extremely complex," a BA spokesperson said.

"We expect around 85 percent of our Saturday Heathrow schedule to run, but it is likely that all travelling customers will experience delays as we continue to navigate the challenges posed by Friday's power outage at the airport."

Restrictions on overnight flights have been temporarily lifted to help ease congestion, according to the UK's transport department. (AFP)