National Grid has insisted its management teams were “not made aware” of problems at a London electricity substation years before a fire in March that led to the closure of Heathrow airport for 24 hours.

Heathrow Airport is considering legal action against National Grid after a report found the fire which caused the airport to shut down was a result of a known fault at an electrical substation. An investigation found that National Grid, which owns the substation which supplies Heathrow, had been aware of a problem since 2018 but failed to fix it.

National Grid (NYSE:NGG) -4.9% in Wednesday's trading after an investigation found the fire which caused London's Heathrow airport to shut down in March, which led to thousands of canceled flights and stranded passengers,