Switzerland's biggest political party yesterday piled fresh pressure on the embattled head of the country's central bank to quit over a currency trade scandal.

Swiss media said Philipp Hildebrand had failed to defuse the crisis when he pledged at a news conference on Thursday to fight accusations of wrongdoing over the controversial trade by his wife Kashya and refused to step down.

She spent 400,000 Swiss francs ($420,000) to buy dollars last August, just three weeks before the Swiss National Bank (SNB) imposed a cap on the soaring Swiss currency.

Hildebrand, a 48-year-old former hedge fund executive, said he learned of the trade the following day.

The Swiss People's Party (SVP), a vocal critic of Hildebrand and the SNB's intervention in foreign exchange markets under his charge, called for a special parliamentary session to examine the case.

"It is unlawful and completely untenable that leaders of the SNB carry out currency actions also in their private affairs. Philipp Hildebrand is no longer acceptable as chairman of the Swiss National Bank," the SVP said.

In practice it would be hard for the SVP to get a special session as they would need to secure a majority in the lower and upper houses.

Swiss tabloid paper Blick ran a picture of Hildebrand leaving an up-market restaurant in Zurich with his wife and daughter after the Thursday Press conference.

Other political parties appeared satisfied with Hildebrand's performance and his promise to improve transparency at the central bank.

The government has also backed Hildebrand, who has been under intense pressure since a Bank Sarasin employee leaked bank account details of him and his wife, showing the family bought foreign currency very close to the time the SNB capped the franc.

Switzerland guards bank client confidentiality jealously and the employee is also in the firing line.