The Swiss people approved by a very narrow majority to spend six billion Swiss francs ($7.5 billion) to modernise the Alpine nation's air force, with the current McDonnell Douglas F/A-18 Hornets set to come to the end of their service in 2030.

"Given the foreseeable additional costs, it is not financially possible to maintain the initially planned number of 36 F-35As," the Swiss government said in a statement.

The government has therefore instructed the Swiss defence ministry "to acquire as many F-35A aircraft as the funding approved by the Swiss public allows, namely six billion francs", the statement said.

Swiss defence minister Martin Pfister told a press conference in Bern that "we do not know exactly" how many jets that will be, as it would depend on the US inflation rate.

When setting out the case for the F-35, the Swiss government insisted that the US-made jet was by far the best and available at a lower price than its made in Europe Eurofighter or Rafale competitors.

Bern also claimed that the jet's price was set in stone and would not move, citing American assurances on the matter.

But on June 25 the Swiss government said that the United States was pointing to additional costs of between 650 million and 1.3 billion Swiss francs linked to inflation, changes in raw material prices and other factors.

"The discussions held this summer with the United States showed that Switzerland could not enforce the contractually agreed fixed price," the statement on Friday said.

Switzerland and the United States are meanwhile locked in negotiations on the details of a deal slashing a whopping 39 percent levy threatened by President Donald Trump to 15 percent, which left Bern scrambling to avoid a duty that ranked among the highest in Trump's tariff blitz.