The chief executive of Heathrow has defended the running of the airport after a fire at an electrical substation stopped about 1,300 planes and disrupted the journeys of hundreds of thousands of global passengers.

It comes after Willie Walsh, the International Air Transport Association’s director general, criticised Heathrow for the disruption.

“This is yet another case of Heathrow letting down both travellers and airlines,” Walsh said.

Speaking to the Radio 4 Today programme on Saturday, Heathrow’s boss, Thomas Woldbye, said that while “lessons needed to be learned” from the fire on Friday, most other airports also operated without a separate power plant.

He added: “Our systems are not designed to power the entire airport, [for that] we would need a separate standby power plant on the side and we don’t have that.

“Heathrow uses as much energy as a city every single day so we don’t have backup power for baggage systems, fuel systems, things like air bridges and so on.

“So whereas the safety systems are working and we can get aircraft in and out most of the airport infrastructure comes to a standstill when we ned to reset as we did yesterday. That’s how most airports operate.

“We are going to, of course, look at what we can learn from this incident but the incident yesterday was a major one, it was not a small thing.”

British Airways, which has a major presence at Heathrow, said it expected to operate about 85% of its scheduled flights at the airport on Saturday but huge delays were still expected.

The airline would usually expect to run nearly 600 departures and arrivals on a Saturday but it is understood cancellations will be made, where possible, to high-frequency routes.

Meanwhile, Virgin Atlantic issued a statement on Saturday apologising for the disruption and said it planned to run a “near full schedule with limited cancellations today”.

The airline said Heathrow’s closure on Friday had left an “impact on today’s flying programme”, including the repositioning of aircraft and crews.

Downing Street said there were “questions to answer” about the fire, with counter-terrorism police leading the investigation into the “unprecedented” incident that left Britain’s biggest airport unable to function.

Two people familiar with the investigation said officials did not believe the fire was the result of any criminal activity or an act of sabotage by a hostile state, and was more likely to be accidental.

Police confirmed on Friday evening that the fire was not thought to be suspicious.

Asked about an inquiry and whether National Grid had questions to answer, the prime minister’s official spokesperson said: “There are questions to answer on how this has happened and what can be done to prevent the scale of disruption we’ve seen from happening again, once the situation is under control.”