Air travel chaos that caused delays for more than 700,000 passengers was fuelled by a “lack of planning” and engineers working from home, investigators have said.

Hundreds of flights were cancelled and thousands of people were stranded abroad by an air traffic control (ATC) system fault on 28 August last year.

It was Bank Holiday Monday, one of the busiest days of the year for air travel.

Airlines lost around £100m in refunds, rebookings, hotel rooms and refreshments after air traffic control provider National Air Traffic Services (Nats) suffered a technical glitch while processing a flight plan.

The report estimates that more than 300,000 people suffered cancellations, while approximately 95,000 endured delays of over three hours, and at least a further 300,000 were hit by shorter delays.

An interim report, published on Thursday by the regulator the Civil Aviation Authority (CAA), said an inquiry panel found no “multi-agency rehearsal of the management of an incident of this nature and scale”.

It’s the sort of dry run that’s a regular feature of planning in other sectors, the panel said.

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The report said there was “a significant lack of pre-planning and co-ordination for major events and incidents” focusing on how to put things right.

Nats wouldn’t usually schedule planned maintenance work on public holidays, the report said, so engineering staff would normally “be available on standby at remote locations – typically at home”.

It took 90 minutes for a Level 2 on-call engineer to get there and carry out a full system restart which couldn’t be done from home, but still, no one called for a more senior engineer “for more than three hours after the initial failure”.

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The flight plan in question listed two waypoints, or locations, with the same abbreviations, causing the system to generate a “critical exception error” and shut down to “prevent the transfer of apparently corrupt flight data to the air traffic controllers”, the report said.

Many affected passengers had to pay up front for alternative flights, food and accommodation – and submitted claims to airlines for reimbursement – despite the carriers being legally required to provide these.

The financial cost to passengers was “very considerable”, but the panel noted that the “stress and anxiety” was “at least as serious”.

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Tim Alderslade, chief executive of Airlines UK, which represents UK-registered carriers, said the report contains “damning evidence that Nats’ basic resilience planning and procedures were wholly inadequate and fell well below the standard that should be expected for national infrastructure of this importance.”

Some travellers were stranded overseas for several days.

A Nats spokesman said it had already been working on improvements.

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