Online flight tracking service FlightRadar24 said the closure would affect at least 1,351 flights to and from Heathrow. It said 120 flights to the airport were in the air when the closure was announced.
It is reported that around 50 flights between Heathrow and Irish airports will be impacted by the closure.
Heathrow is Britain's largest airport, with more than 83.9 million passengers travelling through its terminals in 2024.
A Heathrow spokesperson said: “Heathrow is experiencing a significant power outage across the airport due to a large fire at a nearby electrical substation. Whilst fire crews are responding to the incident, we do not have clarity on when power may be reliably restored.
“To maintain the safety of our passengers and colleagues, we have no choice but to close Heathrow until 23h59 on 21 March 2025. We expect significant disruption over the coming days and passengers should not travel to the airport under any circumstances until the airport reopens.
“We will provide an update when more information on the resumption of operations is available. We know this will be disappointing for passengers and we want to reassure that we are working as hard as possible to resolve the situation.”
At least six flights diverted from Heathrow will land at Shannon Airport.
A spokesperson for the Shannon Airport Group said: “We are closely monitoring the situation at Heathrow Airport and are providing support by accommodating diverted flights as needed and our team is on-site to assist with the additional passengers.
“So far, Shannon Airport has facilitated six diverted flights originally scheduled to land at Heathrow, including flights from Toronto, Atlanta, Bridgetown Barbados, Boston, Orlando and Newark.
“The first of these flights landed safely at 4.26am this morning.”
The airport has advised passengers that all London Heathrow flights to and from Shannon Airport today have been cancelled.
"Intending passengers are advised to contact their airline. Aside from this, Shannon Airport is operating its normal flight schedule today," the spokesperson added.
Online tracking services showed other flights being diverted to London's Gatwick Airport and Charles de Gaulle Airport in Paris.
A number of flights were also turned around and returned to airports in Canada and the United States.
Gatwick Airport confirmed it had accepted seven diverted flights from locations including Singapore, Johannesburg, Lagos, Cape Town and Doha which were originally destined for Heathrow.
“We are aware of the situation at Heathrow Airport today and we are supporting by accepting diverted flights as required,” an airport spokesperson said.
“Flights are operating from London Gatwick as normal today.”
The cause of the fire is unknown and the chairwoman of Britain's House of Commons transport committee said it is “speculative” to suggest at the moment that something sinister caused it.
Asked by Times Radio if she thought the fire at an electricity substation may have been caused intentionally, Labour MP Ruth Cadbury replied: “I think that’s somewhat speculative.
“There are obviously questions about it, and I don’t know enough about electricity, but for the airport to be dependent on one substation, it does raise questions.” She added it was “very, very concerning” that “one substation can close down an airport and there isn’t an alternative source of energy”.
Heathrow Express train service announced it would not operate on Friday while Hayes and Harlington railway station remained closed on Friday morning, National Rail confirmed.
Scottish and Southern Electricity Networks said on X there was a large-scale power outage in Hayes, Hounslow and the surrounding areas impacting more than 16,300 homes.
According to the power company’s website, authorities aimed to restore power by 3pm on Friday.
Emergency services at the scene in Roseville Road, west London, near to the North Hyde electrical substation which caught fire last night. Picture: James Weech/PA Wire
A National Grid spokesperson said the fire had damaged equipment and they were “working at speed to restore power supplies as quickly as possible”.
London Fire Brigade said 10 fire engines and around 70 firefighters were still on the scene at Nestles Avenue in Hayes just before 6am with part of a transformer still alight.
Around 150 people have been evacuated from surrounding properties and a 200-metre cordon has been put in place as a precaution.
London Fire Brigade assistant commissioner Pat Goulbourne said: “This is a highly visible and significant incident, and our firefighters are working tirelessly in challenging conditions to bring the fire under control as swiftly as possible.
“The fire has caused a power outage affecting a large number of homes and local businesses, and we are working closely with our partners to minimise disruption.”
Firefighters led 29 people from surrounding properties to safety.
Footage posted to social media showed huge flames and large plumes of smoke coming from the facility.
The brigade said nearly 200 calls had been received in relation to the incident with crews from Hayes, Heathrow, Hillingdon, Southall and surrounding areas on the scene.
Emergency services were called to the scene at 11.23pm on Thursday. The cause of the fire is yet to be determined.
Mr Goulbourne said firefighters urged people to take safety precautions as crews worked to extinguish the blaze.
“This will be a prolonged incident, with crews remaining on scene throughout the night,” he said.
“As we head into the morning, disruption is expected to increase, and we urge people to avoid the area wherever possible.”
London Ambulance Service and Metropolitan Police both confirmed they were on scene supporting fire crews. Manon Gilbart, Jessica Coates and Rob Freeman, PA, Irish Examiner
