A row's broken out between Ryanair and Air Traffic Control, with the airline claiming provider NATS discriminates against planes using Stansted Airport.
Ryanair says priority is given to aircraft using Heathrow and Gatwick and has submitted a formal complaint to the European Commission.
NATS says it can't be blamed for Ryanair's poor performance over the summer.
Ryanair claims 52% of all ATC delays in the London area "caused by NATS" during the first three months of the year affected flights at Stansted, which is its largest base.
This is compared with zero at Heathrow and 10% at Gatwick.
A failure of NATS to "fairly supply" staffing and airspace resources at Stansted has "wreaked havoc" this summer, according to Ryanair.
Ryanair has submitted a formal complaint to the European Commission and the CAA over the issue.
NATS is owned by a public private partnership including the Government (49%) and a group of UK airlines (42%) such as British Airways and Virgin Atlantic.
NATS insisted that it does not discriminate between airlines or airports and declared that Ryanair's performance this summer "cannot be blamed on UK air traffic control".
It said in a statement that the figures quoted by Ryanair coincide with the introduction of new technology that affected flight capacity at Stansted and other airports such as London Luton over a seven-month period.
Manchester Airports Group, which owns Stansted, say they are seeking seeking answers from NATS on the root cause for these delays and considering the need for further action, including the possibility of making a formal complaint to the CAA in due course.