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.

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,

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.