Ryanair has submitted a formal complaint to the European Commission and the UK Civil Aviation Authority over delays at Stansted.
The airline claims air traffic control provider NATS is unfairly discriminating against Ryanair and Stansted – a claim NATS has previously denied.
The move follows publication of the CAA’s Oberon Report, which shows Stansted suffered 52% of all NATS-related delays in the first quarter of 2018.
By contrast, Gatwick had 10% of NATS delays, while Heathrow had 0%. Thirty per cent of NATS delays occured at Luton and 8% at London City.
Ryanair COO, Peter Bellew said: "Ryanair and Stansted are clearly being discriminated against by the UK-airline-owned ATC provider NATS.
"That Stansted has had 52% of all NATS delays in Q1 while Heathrow has 0% (and Gatwick just 10%) is unjustifiable.
"These disruptions are unfair and unacceptable, and we call on the UK Dept of Transport and the EU Commission to take urgent action to ensure that the UK ATC provider (NATS) is fully staffed and treats each London airport fairly."
A NATS spokesperson said: "NATS does not discriminate between airlines or airports.
"Ryanair performance this Summer cannot be blamed on UK air traffic control.
"The figures Ryanair quotes from earlier this year coincide with the introduction of new technology that affected the number of flights in and out of Stansted during that period.
"Luton airport was similarly affected at that time and other airports were affected at other times over a seven-month period. All airlines and airports were notified of the timetable in advance and understood the new technology will help us increase capacity safely in the future.
"Ryanair made similar accusations against NATS in the summer 2016. These were thoroughly investigated by the CAA. Their published report stated that: "The CAA has found no evidence that NERL [NATS En Route Ltd] has unduly preferred or discriminated against any party.
"NATS has a duty to ensure commercial aircraft can fly safely through UK airspace.
"Adding extra controllers to the Essex airspace will not make a difference. Additional aircraft cannot fly in that area safely without redesigning the airspace which requires consultation with those affected on the ground."
