A lightning strike temporarily knocked out the fueling system Sunday morning at London Stansted airport, the biggest base for Ryanair Holdings Plc, causing flight delays and cancellations.

Engineers restored the system, though flights may still be delayed, diverted or canceled at the airport about 40 miles (64 kilometers) north of central London, according to a statement on Stansted’s Twitter account.

Stansted is the third-busiest airport in London and a key hub for the discount airline Ryanair. The Irish carrier canceled a number of flights and is advising customers of their options, including a full refund or a free transfer to the next available flight, it said in an emailed statement.

“We apologize to all customers affected by these disruptions, which are entirely beyond our control,” Ryanair said in the statement, declining to quantify the cancellations.