"Although one-off effects such as tax amnesties are unavoidable, we ourselves must not give clients any reason to leave the bank," Oswald Gruebel says in the memo.

The Zurich-based bank has struggled with withdrawals from its private banking arm for a variety of reasons, including major write-downs on illiquid real estate securities and a drawn-out US investigation into offshore accounts of UBS clients. It has also suffered staff defections. For example, in 2008, a team of more than 50 employees in London left to help create the new UK start-up, Vestra Wealth.

In the three months to the end of June – the latest period for which UBS issued results back in early November, the Swiss bank said its wealth management business continued to suffer outflows of client cash. Net new money outflows were SFr16.7 billion  ($16.4 billion) for Wealth Management & Swiss Bank, SFr9.9 billion for Wealth Management Americas, and SFr10.0 billion for Global Asset Management.

UBS is still the world’s largest international wealth management firm, but in terms of assets it has been knocked off its number one spot by Bank of America/Merrill Lynch, although the latter firm remains a predominantly domestic US player.

Reports said that Mr Gruebel’s memo is devoted to detailing a new code of conduct and ethics for UBS's private bankers.

"What has happened over the past few years must no longer be possible," Mr Gruebel was quoted as saying, alluding to the recent US investigation into claims that UBS helped wealthy US citizens evade taxes.

On 9 February, UBS is due to report fourth-quarter earnings.