"This is an estimation that covers possible accords in the whole of Europe," UBS chief executive Sergio Ermotti told Swiss newspaper NZZ am Sonntag in an interview.
"In the worst case scenario, another 30 billion francs ($32.7 billion) could flow out," he said.
However, Ermotti stressed that the sum would make up just "two percent of funds, that we are trusted with in asset management."
Switzerland has come under intense pressure over its banking secrecy rules after the financial crisis, as industrialised economies depleted of public funds seek to claw back revenues from tax cheats.
It has since done deals with Britain and Germany to regularise assets hidden by the two countries' taxpayers in Swiss banks.
The accords require Swiss banks to make a cash payment, which would be returned if their clients voluntarily come forward to settle their tax dues.