The Swiss government will sign a contract by the end of March to buy 36 Lockheed Martin F-35A Lightning II fighter jets, it said on Wednesday, pressing ahead without waiting for a referendum on the $5.5 billion deal.

Switzerland chose the F-35 last year as its next-generation fighter jet, angering opponents who said they would ensure a referendum was held to overturn what they called an unnecessary "Ferrari" option.

The government said it was unclear whether the purchase could take place under the same conditions if the deal had to be renegotiated once the offer expires next year.

Many countries have stepped up spending on weapons systems, it said, saying Finland had decided to buy 64 F-35As, Germany wanted to purchase up to 35 and Canada would buy 88.

"Against this background, the risk of a delay in delivery has increased significantly. However, the aircraft must be delivered as planned from 2027 to ensure that the population is protected from threats from the air after 2030," it said.