Swiss bank UBS AG warned U.S. customers by registered mail their account details may be given to U.S. tax authorities, a method that could itself breach secrecy laws, a Swiss paper said on Sunday.

The use of registered mail and envelopes showing the sender was UBS could enable the U.S. authorities to trace customers wanted for tax evasion well before their details are handed over under a U.S.-Swiss double taxation agreement, Sonntag weekly paper said.

A spokesman for UBS declined to comment on the report.