Eleven of Prestwick’s remaining routes next summer are to Spain, with the remainder to Italy, Greece, Portugal and Malta
Mr O’Leary said the airline now only covered its costs at Prestwick, after previously making “quite a bit of money” there, and warned that if it started making losses it could pull out.
He said: “We are a break-even operation there. If it starts to lose money, we wouldn’t think twice of leaving.”
He added that Ryanair’s major maintenance base at the Ayrshire airport would remain “whether we fly to Prestwick or not”.
However, Mr O’Leary said the airport provided a competitive edge for Glasgow’: “Prestwick has lower costs and It keeps Glasgow honest”.
Mr O’Leary declined to express a view on Scottish independence but said the scrapping of Airline Passenger Duty (APD) - the SNP has pledged to at least reduce it - was crucial, regardless of the referendum vote.
Passengers pay £13 in APD on flights from UK airports on short-haul routes which Ryanair operate.
He said: “It is a major disincentive to visitors coming to Scotland and one of the reasons we have struggled to grow the business at Prestwick.
“The future for Prestwick remains challenging while you are under the yoke of APD.”
He said Glasgow Airport would “continue to be a focus area for our growth” and it offered the same growth potential as Edinburgh, where Ryanair has 32 summer routes.
He said: “The range of services at Glasgow is pretty narrow. There is not enough spread of flights to other European cities.
“Glasgow Airport has seen how fast we have grown at Edinburgh and want to share in that growth.”
Ryanair increased its routes from two to 23 in 2008.