LONDON — Britain’s energy regulator on Saturday demanded answers about the country’s biggest electrical blackout in more than a decade, which affected about a million homes and left commuters stranded on packed rush-hour trains in unlit tunnels.

In a statement, the regulator, Ofgem, said it had “asked for an urgent detailed report from National Grid,” the company that operates Britain’s electricity supply network. The regulator, which can fine energy companies if it believes they have been in breach of their license to supply energy or have violated consumer protection law, said it might begin enforcement action.

The director of National Grid, meanwhile, said the company was certain the power failure had not been the result of “malicious” action or a cyberattack.

Power cut off in scattered locations around the country shortly before 5 p.m. Friday, and the blackout lasted for less than an hour. At least one airport, in Newcastle, was affected, along with several rail lines, including one of the country’s two main north-south routes and Thameslink commuter services in London.