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.

Switzerland and the United States settled a row over evasion of U.S. taxes in August when Switzerland agreed to hand over details of 4,450 U.S. accounts at UBS.

But it could take into early 2010 before the first names are handed over under the agreed legal procedures.

Some 7,500 Americans voluntarily disclosed information about hidden overseas assets under a tax amnesty program that expired on October 15, according to the top U.S. tax collector.

UBS had agreed in February to pay $780 million to settle a criminal investigation accusing it of helping American clients evade taxes. At the same it agreed to release the names of about 250 clients in a first breach of Switzerland's banking secrecy.

Sonntag quoted lawyer Andreas Rued, who is representing some U.S. clients of the bank, as saying the use of registered mail and envelopes showing the name of the bank could constitute a contravention of Switzerland's banking secrecy laws. He said he was considering whether to seek a criminal investigation against the bank.

No one was immediately available at Rued's Zurich office.

(Reporting by Jonathan Lynn, editing by Will Waterman)