The prime minister’s spokesman says “there are questions to answer” over how a fire knocked out Heathrow.
Heathrow Airport will remain shut until midnight after a large fire at a nearby electricity substation, disrupting travel for tens of thousands of passengers.
Counter-terror police are among agencies investigating the cause of the fire, which is expected to affect more than 1,300 flights today.
It is not yet known what caused the fire at the substation, but emergency services were first called to Nestles Avenues in Hayes, west London at 23:23 GMT on Thursday.
The London Fire Bridge (LFB) said the fire was under control by 06:28.
Heathrow is one of the world’s busiest airports and had a record 83.9 million passengers last year, with a plane landing or taking off around every 45 seconds.
The National Grid, which says an “interim” solution is in place to restore power to Heathrow.
In a statement, the company says it has “reconfigured” its network to restore power to affected homes as well as “parts of Heathrow airport”.
It adds: “This is an interim solution while we carry out further work at North Hyde to return the substation and our network to normal operation.
“We are continuing to work closely with all stakeholders to manage this incident, and are focused on returning to normal resilience levels as soon as possible.”
It also apologises again for the disruption caused.
Power appears to be back on at Terminal 4, with lights are on in the main building and lifts are working in the multi-storey car park as well.
It was one of two terminals, along with Terminal 2, that had been left without power after initial efforts this morning.