A power outage forced Heathrow Airport to close for a number of hours today, resulting in major disruption for passengers flying to and from Europe's busiest airport.
The power loss was caused by a blaze at an electricity substation in Hayes, west London, late yesterday evening.
Here is what we know about what went wrong:
What caught fire?
A transformer at the substation caught fire.
London Fire Brigade Deputy Commissioner Jonathan Smith said: "The fire involved a transformer comprising 25,000 litres of its cooling oil fully alight.
"This created a major hazard owing to the still live high voltage equipment and the nature of an oil fuelled fire."
What caused the fire?
It is not currently known what caused the blaze.
The Metropolitan Police said they are not treating the incident as suspicious, although the force warned that inquiries are ongoing.
Following that initial confirmation from the Met, the LFB announced its investigation will focus on the electrical distribution equipment.
How did the fire lead to Heathrow's power outage?
Heathrow's chief executive Thomas Woldbye explained to reporters that the airport has three substations, each with a backup transformer.
The fire broke out at one of the three substations where the backup transformer also failed, causing a loss of power to Heathrow.
Why did the airport have to close?
Mr Woldbye said that the airport can run on power from the two unaffected substations but that they had to "restructure the supply".
"So we're not out of power but we have to restructure our power supply," he told the media.
"To do that we have to close down systems - that is safety procedure, we will not go around that."
He added: "Two substations can run the airport but we need to re-engineer the structure of the power supply for all the terminals and that's what we were doing during the day, and then we have to restart all the systems and that's what we've done, and we now see operation coming back."
Further flight disruptions possible
Dublin Airport warned that further disruptions to flights are possible over the coming days.
More than 1,000 flights to and from the major London airport were cancelled as a result, with hundreds of thousands of passengers affected in the disruption linked to the fire.
These included 34 flights to and from Dublin Airport, 14 at Belfast City Airport and six at City of Derry Airport.
Shannon Airport has facilitated six diverted flights which had originally been scheduled to land at Heathrow, involving flights from Toronto, Atlanta, Bridgetown Barbados, Boston, Orlando and Newark.
Dublin Airport said in a statement that further disruption to flights between Dublin Airport and Heathrow over the coming days is possible.
