Getty ImagesThe boss of Heathrow Airport has apologised to the more than 200,000 passengers whose journeys were disrupted by its closure last month following a fire.
Thomas Woldbye offered his "deepest regrets" adding that the "situation was unprecedented".
Speaking to MPs, he said he recognised "the considerable inconvenience and concern it caused".
The airport was shut down for more than a day in March after a fire at a nearby electrical substation.
Mr Woldbye said Heathrow realised "during the early hours" of Friday 21 March that "we were losing power to the airport".
"In our operations centre you would seen all the red lights go, that the systems were powering down," he said. "We had no information as to why."
"We then had a slightly later stage call from the fire department that the substation was on fire," he said.
Heathrow is supplied by three substations, but knocking out one caused loss of power to the airport.
Mr Woldbye said a third of the airport was powering down and that Terminal 2 was particularly affected. He added that it became "first and foremost a safety situation".
"We need to make sure, when a crisis happens, that people are safe," he said.
