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,

Regulator Ofgem today said it will launch a further investigation of the missed opportunities by National Grid to maintain the substation properly and could take enforcement actio