Credit Suisse could see clients in western Europe withdraw up to a net $37 billion (23 billion pounds) in the next few years as Switzerland bows to pressure to stop foreigners using secret offshore accounts to evade taxes.
Swiss bank secrecy - which has helped the country build a $2 trillion offshore financial centre - has come under heavy pressure in recent years as cash-strapped governments have sought to fight tax evasion.
In a webcast of a presentation to a conference in New York, Finance Chief David Mathers said Credit Suisse had already seen more than 30 billion francs ($31.98 billion)in net outflows from mature offshore markets since 2009, part of it due to the tax disputes.
"Cross-border transformation including new tax treaties could result in 25-35 billion francs outflows over the next few years," the bank said according to slides for the presentation.
Yet Mathers said the industry outlook remained "highly attractive", with strong asset inflows from emerging markets and into the bank's onshore Swiss and international booking centres more than compensating for these losses.
Last year, the bank saw inflows into those segments of 45 billion francs, while the offshore mature markets unit recorded outflows of 8 billion francs.
Credit Suisse wealth management had 774 billion francs in assets under management as of end June.
Switzerland has struck deals with Germany, Britain and Austria to tax their citizens' accounts without revealing their identities, which it hoped would be blueprints for other countries in Europe, including Greece and Italy.
But the deal with Germany - the biggest market for Swiss banks in Europe - is under threat from the opposition Social Democrats who see it as too lenient on tax evaders.
Last year, Credit Suisse paid a fine of 150 million euros to end an investigation over allegations the bank and its employees helped rich Germans dodge taxes.
Pressure on the private banking business comes as Credit Suisse's investment bank has also foundered, forcing it to announce deep cost cuts and a bundle of measures to boost its capital in response to concern from the Swiss National Bank.
Among those steps, Mathers said Credit Suisse should be in a position to make announcements before the end of the year on plans for 1.1 billion francs of strategic divestments.
He also expects gains to be booked in 2012 from 500 million francs of planned real estate sales, with firm offers received for two major sites and other disposals close to signing.
(Reporting by Catherine Bosley and Emma Thomasson; Editing by Hans-Juergen Peters)