A spokesman for the bank told SonntagsZeitung newspaper UBS had stopped dealing with Iranian individuals, businesses or state institutions since the beginning of the year, though Iranian exiles were not affected.

The bank refused to be drawn on whether the decision was directly linked to the dispute over Iran’s nuclear programme.

The process leading to the termination of relations with Iranian clients had already begun last year, according to UBS.

Swiss radio reports said the bank was considering similar measures against Syrian customers.

UBS said it had decided it no longer had any interest in doing business with Iran, but that the rising tensions over Iran’s nuclear ambitions were only one factor in that decision, the spokesman said.

Meanwhile, the country’s second-largest bank Credit Suisse was monitoring developments in the dispute over Iran “with growing concern”, but the bank could not simply opt out of import-export financing, a spokesman for that bank told the paper.

According to SonntagsZeitung, both banks are heavily engaged in the US market.