The UK government has initiated an urgent inquiry into the power failure that led to Heathrow Airport's closure for almost a full day.

The National Energy System Operator will lead the investigation, which seeks to identify the outage's cause and evaluate the resilience of the nation's critical infrastructure.

In the meantime, Heathrow Airport has appointed an independent board member to review the airport's crisis-management strategies and its response to the situation.

Operations at the airport have restarted, with more than 1,300 flights scheduled for Sunday.

However, airlines are cautioning passengers about potential further delays and cancellations as they strive to recover from the disruption.

British Airways, which primarily operates out of Heathrow, has vowed to operate a 'near-full' schedule for Sunday.

In addition, major airlines are allowing changes to fares for those affected by the outage without penalties.

The outage originated from a fire at an electrical substation near the airport, resulting in a power loss that necessitated the diversion of planes to alternative airports.

Around 70 firefighters and 10 fire engines were deployed in response to the incident, which authorities do not consider suspicious.

The London Fire Brigade will focus its investigation on the equipment used for electrical distribution.

Heathrow Airport ranks as the world's fifth busiest airport, managing over 1,351 flights and approximately 291,000 passengers on a typical day.