Authorities in the United Kingdom have ordered an inquiry into a power outage that shut down operations at London's Heathrow Airport.

The outage, which was caused by an accidental fire at an electrical substation on Friday, shut down the world's fifth-busiest airport for almost a day.

Over 1,300 flights were canceled and around 200,000 passengers were stranded on Friday.

Flights resume but delays expected to last for days

Although officials said Saturday that the airport was once again "fully operational," thousands of travelers were still stuck due to the severe disruption caused by the outage. The delays are expected to last for several days.

British Airways, which is based in Heathrow, said it hoped for a "near-full schedule" on Sunday, although the airline's chief executive Sean Doyle warned Friday that the outage's "huge impact" could continue into next week.

The British government has said it has ordered the National Energy System Operator (NESO), which is in charge of the country's electricity and gas grid, to investigate the incident.

"I have today commissioned the NESO to carry out an investigation into this specific incident and to understand any wider lessons to be learned on energy resilience for critical national infrastructure, both now and in the future," Energy Secretary Ed Miliband said in a statement on X.

The investigation is expected to report initial findings within six weeks.

"We are determined to properly understand what happened and what lessons need to be learned," Miliband said.

Heathrow Airport has also opened its own inquiry into the accident, which will be led by Ruth Kelly, a former UK transport secretary and member of the airport's board.

According to aviation experts, the last time European airports faced such major disruption was when an Icelandic volcano erupted in 2010, sending a plume of volcanic ash into the sky that forced some 100,000 flights to be grounded.

Edited by: Wesley Dockery