An investigation has been launched into the closure of London Heathrow Airport on Friday (21 March) due to a power outage that was caused by a major fire at an off-site electrical substation.

Heathrow had to suspend all flights for around 18 hours because of the loss of power before services were eventually able to resume around 4.30pm.

The closure caused massive disruption to passengers on Friday when more than 1,000 flights were cancelled, although Heathrow said it had operated a full schedule on both Saturday and Sunday (22-23 March).

The airport said in an online statement on Monday (24 March): ¡°Heathrow is open and fully operational. Teams across the airport continue to do everything they can to support passengers impacted by Friday¡¯s outage at an off-airport power substation.¡±

UK energy minister Ed Miliband has launched an investigation into the power failure at Heathrow, which will be carried out by the independent National Energy System Operator (NESO).

¡°NESO¡¯s investigation will support efforts to build a clear picture of the circumstances surrounding this incident and the UK¡¯s energy resilience more broadly so that it¡¯s prevented from ever happening again,¡± said the UK government in a statement.

Miliband added: ¡°The loss of power to the Heathrow area has caused major disruption to thousands of people and many businesses. We are determined to properly understand what happened and what lessons need to be learned.¡±

NESO, which is expected to report its initial findings within six weeks, will work with other bodies, including Heathrow, to ¡°ensure the investigation is comprehensive and thoroughly examines the causes of the incident¡±.??

Heathrow CEO Thomas Woldbye said: ¡°We are committed to working closely with all stakeholders to ensure a thorough investigation to help strengthen the airport's future resilience.¡±

The airport has also started an internal review into its crisis management plans following the incident, which will be led by former transport secretary Ruth Kelly, who is currently an independent member of Heathrow¡¯s board.

Heathrow¡¯s chairman Paul Deighton said that the airport¡¯s closure had caused ¡°significant impacts for our passengers, our customers, our colleagues and the country¡±.

"Heathrow regrets the disruption this caused. We hope that all those affected understand that the decision was made in order to prioritise the safety of our passengers and colleagues,¡± added Deighton.

"We are committed to finding any potential learnings from this unprecedented incident."