ZURICH, Feb 7 (Reuters) - The United States is unwilling to re-enter talks to alter a key deal struck with Switzerland to end a damaging tax case against Swiss banking group UBS AG , the U.S. ambassador in Berne was quoted saying in a Swiss Sunday paper.

'The United States does not want to renegotiate. It has a perfect agreement,' Donald Beyer was quoted as saying by Sonntag, adding the United States had received repeated reassurances from Berne that it would do everything in its power to honour the agreement.

'Now it is all about implementing it under Swiss law. That is a thing for the Swiss government not for the United States,' Beyer said.

A Swiss court last month ruled in favour of a UBS client seeking to prevent her account data from being given to the U.S. tax agency, throwing doubt on Switzerland's ability to deliver details of most of the 4,450 UBS client accounts as agreed in August.

The Swiss government said it was sure it could resolve the legal impasse stopping it from handing over the data on clients the U.S. suspects of dodging taxes, either by negotiating with the United States or if necessary by forcing a parliamentary vote on the treaty.

Parliamentary backing would be the best way to secure the treaty, Beyer said when asked if the Swiss parliament should ratify the treaty, adding not finding a solution was unimaginable.

The United States has said it would withdraw the summons seeking client names if it got 10,000 UBS account holder names voluntarily, providing these included the 4,450 names it was seeking under the terms of the treaty.

Beyer said it was unlikely so many UBS clients had come forward in a U.S. tax amnesty, despite newspapers reporting 14,700 U.S. tax payers had declared themselves.

'It is absolutely unlikely that 10,000 UBS clients are among those,' he said.

'The agreement between UBS and the United States is already active. It does not stand and fall on the number 10,000' he added.

Separately the Swiss finance minister said in an interview published on Sunday Switzerland must consider the automatic exchange of tax information with European Union governments if Swiss banks are to have unlimited access to EU markets.

(Writing by Jason Rhodes; Editing by David Holmes) Keywords: UBS/US

(jason.rhodes@thomsonreuters.com; +41 58 306 7312; Reuters Messaging: jason.rhodes.reuters.com@reuters.com)

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