It comes as flights around the world have been cancelled or diverted due to a fire at a nearby electrical substation.
The blaze at North Hyde substation in Hayes, west London, which supplies Heathrow, caused a major power outage at the airport.
It is not yet known what caused the fire, but emergency services were first called to Nestles Avenues in Hayes after 11pm on Thursday (March 20).
The London Fire Bridge said the fire was under control by 6.28am, BBC News reports.
Posting on X (formerly known as Twitter) about the situation, Sir Keir thanked emergency workers and shared he was receiving regular updates.
He wrote: “I know the situation in Heathrow is causing distress and disruption, especially for those travelling or without power in their homes.
“I’m receiving regular updates and I’m in close contact with partners on the ground.
I know the situation in Heathrow is causing distress and disruption, especially for those travelling or without power in their homes.
I’m receiving regular updates and I’m in close contact with partners on the ground.
Thanks to our emergency workers for keeping people safe.
— Keir Starmer (@Keir_Starmer) March 21, 2025
“Thanks to our emergency workers for keeping people safe.”
More than 1,300 flights have been affected, tracking website Flightradar24 said, BBC News adds.
Many people have been stuck at airports in other countries, and flights already on their way to Heathrow have been diverted or returned to their host airport.
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How long will Heathrow Airport be closed?
The airport will be closed until at least 11.59pm on Friday, Heathrow said in a statement, but it warned customers to expect “significant disruption over the coming days”.
The travel disruption was expected to affect many other airports, as airlines cancelled and diverted flights.
Heathrow said it does “not have clarity on when power may be reliably restored”.
The airport has apologised for the disruption and has advised passengers to contact their airlines for further information.
