Passengers warned of delays as flights set to resume at London's Heathrow Airport

London's Heathrow Airport said it was "safely able to restart flights" Friday afternoon after a fire at a nearby electrical substation closed one of the world's busiest airports for nearly a day.

However, even as the airport said it could restart flights, officials advised people not to travel "unless your airline has advised you to do so." The airport said it hopes to "run a full operation" on Saturday.

"Our teams have worked tirelessly since the incident to ensure a speedy recovery. We're now safely able to restart flights, prioritising repatriation and relocation of aircraft," Heathrow said in a statement.

The fire at the substation caused a "significant power outage" to the airport, which was initially expected to be closed until 11:59 p.m. local time on Friday night, authorities said.

"We expect significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens," a spokesperson for Heathrow Airport said.

Impact on flights

In a statement Friday, British Airways said it expected to operate around 85% of the nearly 600 arrivals and departures scheduled Saturday but said all passengers should be prepared for delays.

"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," the airline said in a statement.

An analysis said as many as 290,000 passengers were expected to be impacted by the closure Friday, according to Cirium, an aviation analytics company. There were 669 flights scheduled to depart to Heathrow on Friday, with 145,836 seats, according to Cirium. The fire could affect another 270,000 passengers on Saturday, too, Cirium said.

"Accordingly, the impact of this incident can cascade over several days, as aircraft, crew, passengers are out of place, with limited spare aircraft and seats available to recover passengers," Cirium said in a statement.

Earlier Friday, inbound flights to Heathrow were being redirected to other U.K. airports, officials said.

"Due to a power outage in the London Heathrow area, London Heathrow Airport is currently closed," British Airways said. "As a result, customers due to travel from Heathrow on Friday are advised not to travel to the airport until further notice. This will clearly have a significant impact on our operation and our customers and we're working as quickly as possible to update them on their travel options for the next 24 hours and beyond."

Fire investigation continues -- but not seen as suspicious

The blaze at the West London power station sent plumes of smoke billowing into the sky, led to a large number of power outages in other homes and businesses and led to some evacuations of residents in nearby properties, according to the London Fire Brigade.

Approximately 5% of the fire "remains alight in isolated hotspots" at the substation, the London Fire Brigade said in a statement Friday night.

"The fire involved a transformer comprising 25,000 litres of its cooling oil fully alight," London Fire Brigade Deputy Commissioner Jonathan Smith said in a statement Friday. "This created a major hazard owing to the still live high voltage equipment and the nature of an oil fuelled fire. As of the most recent update, approximately 5% remains alight in isolated hotspots, and we will maintain a presence at the site until the incident is resolved."

The cause of the fire is still not known.

"After initial assessment, we are not treating this incident as suspicious, although enquiries do remain ongoing," a statement from London's Metropolitan Police said. Because of the location of the substation "and the impact this incident has had on critical national infrastructure," the Met's Counter Terrorism Command was leading the investigation, authorities said.

In total, 70 firefighters and 10 fire trucks responded to the blaze in west London when the brigade was first called at 11:23 p.m. local time Thursday.

Firefighters safely evacuated 29 people from neighboring properties and, as a precaution, established a 200-meter cordon with around 150 people evacuated. Residents were also advised to keep their windows closed due to significant amounts of smoke from the fire.

ABC News' Clara McMichael and Ayesha Ali contributed to this report.