The left-wing Stop F-35 alliance said it had almost secured the 100,000 signatures required under Switzerland's direct democracy system to force an issue to a vote.

It says it does not want the jets because they are costly, and because of a number of technical issues reported with the planes.

They hope to have gathered more than enough signatures to present their initiative to block the jet purchase within the coming months.

It remains unclear when the vote would be put on the agenda in Switzerland, where popular votes on a vast array of issues are held every few months.

"Due to the exploding costs of the F-35, the foreign policy consequences and the many flaws of this combat aircraft, this debate is necessary and urgent", Marionna Schlatter, from the Green party and a member of Stop-35, said in a statement.

A Lockheed Martin spokeswoman told AFP the F-35 was the only aircraft that would meet Switzerland's air defence needs for the next 50 years.

"We remain persuaded that the F-35 is the most modern, the most capable and the cheapest of available aircraft for the Swiss air force's future fighter jet fleet," she said.

The Swiss government in June last year agreed to buy 36 F-35As from US manufacturer Lockheed Martin.

The purchase followed the narrow referendum approval in September 2020 for the military to spend six billion Swiss francs (5.8 billion euros, $6.3 billion) to acquire a new fleet.

The government said the plane was the best, but two Swiss parliamentary committees launched an investigation into why the model was chosen after a series of technical problems reported with the plane in the United States.

They also questioned the high cost of the planes.

But the government said last week it wanted to speed up the purchase process, with the US offer expiring at the end of March 2023, raising questions around whether a vote could take place before then.

The government's rush to complete the process is "highly questionable from a democratic perspective", the Stop F-35 alliance charged, stressing though that it remained possible to hold a vote in time.

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LOCKHEED MARTIN