Swiss, Germans to sign tax deal on Aug. 10-paper

* Swiss paper says banks to pay 2 bln Sfr up front

* Level of withholding tax not yet known-paper

* Banks disagree over how much each should contribute

ZURICH, July 31 (Reuters) - Switzerland and Germany will on Aug. 10 sign off on a deal regularising untaxed money stashed in secret accounts, with Alpine banks paying 2 billion Swiss francs ($2.49 billion) as an upfront guarantee, a Swiss paper reported.

Strict banking secrecy, which helped Switzerland build a multi-trillion dollar offshore banking industry, has come under heavy fire and strained relations between Switzerland and other countries seeking to hunt tax cheats.

Swiss authorities have been in talks with counterparts in both Germany and Britain over tax deals for months. The outlines were agreed upon last year, but one of the main sticking points has been how to deal with legacy wealth, some of which has been stashed away for decades.

Citing "well-informed sources", The SonntagsZeitung said on Sunday that Swiss banks would pay 2 billion Swiss francs as a guarantee that clients with undeclared funds would step forward and pay their tax bills. For Britain that figure would be 500 million francs, the paper said.

Should Germany receive at least 4 billion francs in back taxes, the guarantee money would be returned to the banks, the SonntagsZeitung said.

Germans hold an estimated 200 billion Swiss francs ($202 billion) in untaxed assets in Swiss accounts.

The figure of 2 billion may be good news for the banks; there had been speculation banks could have to cough up more than 10 billion.

UBS , Credit Suisse , Julius Baer , Sarasin , the Swiss Bankers Association and the finance ministry were not immediately available for comment.

Several Swiss banks have been in the crosshairs of foreign tax authorities after UBS had to promise in 2009 to hand over 4,450 account names to settle a U.S. tax probe. Julius Baer paid to settle with German authorities, who have also raided the offices of Credit Suisse. .

NO WITHHOLDING TAX LEVEL YET

One of the other elements of the deals not yet determined has been how high to set the withholding tax on interest and dividend income for future earnings.

In the case of Germany, media reports have suggested a withholding tax could be set as high as 35 percent, though others see the figure of 25 percent as more likely, since that is Germany's tax level for capital gains and profits from share sales.

The SonntagsZeitung said the withholding tax level was not yet known.

Moreover, Swiss banks were still sparring over how to divvy up the 2 billion, it said. Some banks with no offshore business have reportedly been reluctant to contribute to the pool.

The deals will allow Switzerland to preserve the vestige of banking secrecy by foregoing the automatic exchange of information, the paper said.

The banks will also receive immunity from prosecution in Germany and Britain.

The agreements will still have to be ratified by the Swiss parliament.

(Reporting by Catherine Bosley; Editing by Alex Richardson) ($1=.8022 Swiss Franc)