ZURICH (Reuters) - Switzerland's government asked parliament to approve financing for its controversial purchase of 22 Gripen fighter jets and warned the measure could face stiff opposition if put to a referendum.

Switzerland plans to buy 22 JAS-39 Gripens, made by Sweden's Saab, at a cost of 3.126 billion Swiss francs ($3.30 billion) to replace its aging Northrop F-5E/F Tiger fighters.

But the move is unpopular with some because it will require spending cuts in other areas, such as education.

On Wednesday, the government presented a financing package for the Gripen buy, which parliament must now vote upon. The acquisition can also still be derailed by a popular referendum.

Defense Minister Ueli Maurer told a news conference he expected to get a green light from parliament but that convincing citizens would be more challenging.

"In the parliament we're not in the clear yet, but I think the cards are in our favor," said Maurer, a member of the right-wing Swiss People's Party (SVP).

The head of the pro-business Free Democrats (FDP) has indicated he was skeptical about the acquisition; the measure needs the FDP's backing in parliament to pass.

"I regard the people as the biggest hurdle. We will probably vote on it at a time when the economy doesn't have much momentum," Maurer said. "And then people wonder if you should be spending that much on planes."

Switzerland is seeing economic momentum slow, as the debt crisis afflicts countries in the euro zone, its biggest trading partner.

The Green party said in a statement it would seek a referendum if parliament approved funding for the jets, saying the money should instead be used to safeguard the environment.

In Switzerland, a referendum can be held on federal laws if at least 50,000 people or eight cantons have petitioned for one within 100 days of the item's official publication.

($1 = 0.9475 Swiss francs)

(Reporting by Catherine Bosley; editing by Keiron Henderson)