Michael O'Leary has written a strongly-worded letter to the Chancellor, Sajid Javid, saying the government's rescue of Flybe contravenes competition rules.
He argues measures that are being put in place to help Flybe should be extended to other airlines.
If they are not, Ryanair intends to launch legal proceedings against the government, Mr O'Leary said.
Earlier this week, the government approved help for Flybe, which is thought to centre on giving the airline time to pay about £100m of outstanding Air Passenger Duty (APD).
Other airline executives have criticised the move as granting an unfair advantage to ...
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