The Civil Aviation Authority has revealed thousands of airline passengers have been delayed due to sickness and a refusal to work overtime by air traffic controllers.
The finding came as part of an investigation into a complaint made by Ryanair that Heathrow was being prioritised over Stansted.
The CAA rejected the complaint but said the air traffic control, or Nats, which is 49% owned by the government has failed to maintain staffing levels at its HQ in Swanwick, Hampshire.
Ryanair alleged 100,000 passengers had been subjected to delays at Stansted in the first half of 2016 and claimed air traffic controllers gave Heathrow flights priority.
The CAA report founder higher than expected short-term sickness, unplanned retirements and a low rate of overtime take-up due to unfavourable industrial relations.
The report recommended forcing Nats to be “more proactive and transparent” about staffing problems and addressing shortfalls on certain routes.
A Nats spokeswoman told The Times: “The safe air traffic control service that we provide to 2.5 million flights per year is recognised by our customers as one of the best and most resilient anywhere in the world.
“We continually seek to refine our operation particularly in light of the substantial and unforeseen growth of traffic across the southeast of the UK since early 2016.
“We are pleased to confirm that we have already taken a number of the actions referred to in the CAA’s report.”
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CAA rejects claim of air traffic control discrimination against Stansted