Airline chiefs have accused Heathrow of “clear failure” after Europe’s busiest airport was shut down by a fire in a single electricity substation.

The airport closed for nearly 24 hours on Friday in an unprecedented move that left about 200,000 passengers stranded across the globe, with counter-terror police brought in to investigate the source of the fire.

The first flights resumed on Friday night, but airlines have warned the public to expect days of disruption.

Questions are mounting over why there was not an effective back-up power supply to prevent Heathrow closing.

The incident has also led to wider concerns about Britain’s energy security and the power supply to critical national infrastructure amid fears it could be targeted by Russia or terrorists.