The British government has ordered an urgent investigation into how a fire at an electricity substation left Heathrow Airport in London in darkness on Friday, crippling one of the world’s busiest airports.

“We are determined to properly understand what happened and what lessons need to be learned,” Britain’s energy secretary, Ed Miliband, said in a statement late on Saturday.

Friday’s closure disrupted more than 1,000 flights, leaving planes and aviation crews out of position and stranding passengers — some of whom may not reach their destinations for a few more days.

Heathrow said it would operate a full schedule of more than 1,300 flights on Sunday as airlines tried to clear the backlog that has disrupted travel for tens of thousands of people. On Saturday, more than 250,000 passengers passed through the airport “with punctual departures,” the airport said.

The fire, which the authorities believe was likely accidental, raises questions about the resilience of Britain’s key infrastructure and whether the country has invested enough to maintain it.

Britain’s government has faced pressure for years to maintain and modernize the country’s transportation infrastructure, like roads and trains. But the country faces severe financial pressures, with public services like health care underfunded. Any demands for major infrastructure spending would create political headaches for the prime minister, Keir Starmer, while he also tries to increase military spending amid flatlining economic growth.

Within hours of the airport going dark, engineering experts were questioning whether Heathrow was supported by infrastructure befitting a major world hub.

Martin Kuball, a professor of physics at the University of Bristol, wrote in an online post that the fire was a warning sign about the nation’s electrical systems.

“Unfortunately, there is no resilience built into the National Grid,” Mr. Kuball, a Royal Academy of Engineering chair in emerging technologies, wrote at the Science Media Center. “In part, this is because we still rely on old technology in substations that use copper windings to distribute power rather than new technology, so-called solid state transformers.”

The British authorities said they expected preliminary results from the investigation in six weeks.

In a statement, the transport secretary, Heidi Alexander, said that Heathrow “uses the energy of a small city, so it’s imperative we identify how this power failure happened and learn from this to ensure a vital piece of national infrastructure remains strong.”