The management scrutiny committee of the National Council lower house of parliament decided to investigate "the legality and advisability of certain aspects of the evaluation procedure taken for choosing the Swiss military's new combat aircraft", it said in a statement.

A sub-committee will start work in February and publish its findings "in due course".

"We welcome this examination by the parliament and we will do everything to support it," the Federal Department of Defence said in a tweet.

Switzerland's current air defence set-up will reach the end of its service life in 2030 and the search for a replacement fleet was long and hotly contested.

The F-35A combat aircraft -- already used by the US Air Force and several European countries -- was chosen in June this year ahead of the Airbus Eurofighter, the F/A-18 Super Hornet by Boeing, and French firm Dassault's Rafale.

The Swiss government said the F-35A had a marked technological advantage over the other candidates because it had powerful new systems that ensured information superiority.

At five billion Swiss francs ($5.5 billion, 4.7 billion euros), the procurement costs were around two billion francs cheaper than the second-lowest bidder, said the government.

The decision to buy the F-35As could be challenged at the ballot box, with left-wingers and anti-militarists keen to trigger a public vote.

The parliamentary scrutiny committee "wishes to establish transparency on certain points which have aroused criticism from public opinion", it said.

Its sub-committee will focus on the methods used to assess the competing planes, the room for political manoeuvre with the United States and respect for the principles of public procurement rights.

Switzerland is famously neutral. However, its long-standing position is one of armed neutrality and the landlocked European country has mandatory conscription for men.

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LOCKHEED MARTIN

DASSAULT AVIATION

BOEING

AIRBUS GROUP