U.S. authorities have given Swiss banks suspected of selling tax-evasion services to wealthy Americans a Jan 23. deadline to hand over the names of client advisers, a Swiss newspaper reported on Sunday.

Citing banking sources, the NZZ am Sonntag said Swiss banks had until Jan. 23 to transfer documents to the U.S. Justice Department relating to their U.S. business since 2000.

Eleven banks -- including Wegelin, Credit Suisse, Julius Baer, and regional Basler Kantonalbank -- are under formal investigation by U.S. tax authorities.

Switzerland and the United States are negotiating a tax deal to try to get the investigation dropped, in return for expected hefty fines on the banks and the handing over of client names.

Last week, three employees at Swiss private bank Wegelin & Co. were charged by U.S. authorities with helping U.S. taxpayers hide some $1.2 billion in assets from tax authorities.

The Swiss justice department was not immediately available for comment.

"Whether the data is handed over to the U.S. authorities depends on whether the request corresponds to legal requirements," a representative of the Swiss department of justice was quoted as saying in the paper.

(Reporting by Caroline Copley; Editing by Jane Merriman)