Coutts, one of the world's oldest banks that lists Queen Elizabeth II among its clients, comes under the spotlight after Swiss prosecutors began an investigation of British bank HSBC following similar allegations.
   
"A prosecuting authority in Germany is undertaking an investigation into Coutts & Co Ltd in Switzerland, and current and former employees, for alleged aiding and abetting of tax evasion by certain Coutts & Co Ltd clients," RBS said in its annual results statement.
   
RBS is seeking to sell the international arm of Coutts in order to focus on its wealthy clients based in Britain.
   
The RBS statement said Coutts was cooperating with the investigators, and chief executive Ross McEwan told ITV news the bank would take "severe action" if wrongdoing was found.
   
Coutts is also cooperating with the US Department of Justice (DOJ) "regarding the role of Swiss banks in concealing the assets of US tax payers", according to the statement.
   
As part of an agreement to amicably resolve the DOJ's investigation, Coutts collected evidence of the tax status of all US-related account holders and presented the results in November.
   
A resolution was expected this year and provision had been set aside for liabilities resulting from the review, the statement said.