The company has already said it plans to withdraw from Västerås and cut lines at its other Swedish bases if the tax is introduced.
Ryanair's Nordic chief now says that if the European Commission does not rule on the legality of the tax within two weeks it will start its cutbacks immediately.
Speaking to Sydsvenskan, Lotta Linquist-Brosjö said that the company would not wait any longer for the commission's response:
"Our view is that the reply could be delayed further. And we can't wait - we must plan our traffic and have certainty at least six months ahead," she said.
She told the paper that the failure of the commission to reply could lead the company to withdraw from Sweden altogether, and repeated the plan to withdraw from Västerås and to scrap lines from Gothenburg and Malmö.
The flight tax, the product of an agreement between the Social Democratic government and the Green and Left Parties, is being examined in Brussels to see whether it breaks competition rules.
The government has excluded certain routes to remote parts of Sweden from the tax, which could be classed by the EU as illegal state support.
The managing director of Malmö Sturup Airport, Lennart Björk, said he had given up listening to Ryanair's threats.
"I'm so tired of them. They say one thing on one day and another thing the next - it's impossible to speculate. But I believe that in the end they will decide on the basis of how profitable the routes are," he told the paper.