Militarily neutral Switzerland is embroiled in domestic controversy over its planned $6.2 billion purchase of US F-35A fighter jets, prompting defense officials to claim that any delay to the deal would have “grave consequences” for the country’s security.

Wednesday’s statement by the defense department in Bern came one day after the Swiss government confirmed that organizers of a “Stop F-35” initiative had gathered the necessary 100,000 valid petition signatures to force a referendum on the issue. Activists have called for the vote to be held in March, the same month as the government’s deadline to close its 36-jet deal with US defense contractor Lockheed Martin.

The Swiss defense department told the nation’s Federal Council that the proposed timetable for the referendum isn’t feasible because the executive branch and Parliament wouldn’t have enough time to “process” the message sent by voters before the Lockheed offer expires.

“The message must be sent to Parliament by the winter session,” the department said in a statement. “A delay in the acquisition of the F-35A would have grave consequences for the security of Switzerland.”

Part of the concern is that other countries – including Germany, Finland and Canada – are queuing up to buy F-35s amid the Russia-Ukraine conflict. Switzerland could be pushed to the back of the line, and perhaps have to pay a higher price, if it had to start over on a new contract with Lockheed. The Swiss military is racing to upgrade its air defenses by 2030.

However, the “Stop F-35” coalition – which includes Switzerland’s Social Democratic Party, the Green Party and an anti-military group -- has argued that the US-made attack jets are too expensive and not a good fit for the defense-focused Swiss air force. Military neutrality is enshrined in Switzerland’s constitution, so the country’s air force is limited mostly to patrolling the skies in its own region. Critics also have suggested that relying on the F-35A would entangle Swiss national security too closely with Washington.

The Swiss government chose the F-35A last year, after also evaluating France’s Rafale fighter jet, Boeing’s F/A-18 Super Hornet and the Eurofighter Typhoon, which is built by a four-nation group led by Airbus.