Fears of a campaign of cyber attacks on leading airlines have grown after a “systems issue” delayed dozens of United Airlines flights worldwide.

A spokesman for United told The Independent: “We are aware of an issue with our systems that is resulting in flight delays. We are working to resolve the issue and get customers to their destinations as soon as possible. We apologise for the inconvenience.”

United is the world’s third-biggest airline, after Delta and American.

More than 60 flights are known to have been delayed. The first flight from the UK was due out from Heathrow at 7.30am, to Washington DC, but left an hour late. Passengers were told it was “delayed due to operational difficulties”.

The fact that the problem struck late at night in the US meant the impact was less that it would have been 12 hours earlier or later, but many passengers expressed frustration at the delay. Jason Witkowski, a passenger flying from San Francisco to New York, tweeted: “Still here. Still cramped. Still waiting. Still want to get off. UA726 #UnitedAirlines #unitedwewait”. Another passenger in Los Angeles tweeted: “Still on plane waiting on the tarmac after landing over an hour ago at LAX UA887 – this is ridiculous.”

At 3am local time at Newark airport, New York, Roxanne Lott tweeted: “Thanks for all the info United. Way to totally not communicate at all”. But simultaneously, said United said: “The system issue has been resolved. Any delayed flights are resuming.”

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In August, Delta Airlines – the world’s biggest carrier – experienced a worldwide systems failure which led to the cancellation of hundreds of flights and delays to thousands more. The airline blamed a power problem, and denied that its system had been breached, but cyber-security experts were sceptical.

Last month, British Airways passengers were hit with long delays after what the airline called “a problem with our check-in system”.

Airlines are heavily dependent on their computer systems for almost every aspect of their operations, from issuing boarding passes to loading baggage and cargo. While they can revert to manual operations, the processes involved can cause severe delays which in turn lead to cancellations.