A German tax inspector is in talks to buy banking data from Switzerland to help his office identify tax evaders, a magazine reported on Saturday - days after the two countries signed a landmark deal on taxing secret deposits.

The move seems likely to stoke bilateral tensions before Germany's parliament votes on Thursday's agreement, which is designed to stop wealthy German tax dodgers holding cash in secret Swiss bank accounts.

Weekly Der Spiegel said the head of the tax inspectors office in Wuppertal in the state of North Rhine-Westphalia was in negotiations to buy two new sets of data from Switzerland.

Swiss authorities caused an outcry in Germany last weekend when they said they had issued arrest warrants for three German tax inspectors, accusing them of industrial espionage for buying bank details of German tax evaders.

Thursday's agreement - under which Switzerland will tax Germans' accounts and pass the proceeds to Germany - was in part a consequence of similar purchases in 2010 by several German states which put pressure on Switzerland to change its tradition of banking secrecy.

Some of the data in the latest case concerns internal information from Coutts private bank in Zurich, owned by Royal Bank of Scotland and the seller wants about 2 million euros for data on about 1,000 customers, the magazine reported.

As in previous cases, the finance ministry would cover half the costs, it said.

A ministry spokesman said there had been repeated offers of information and each one was looked at carefully. He would not comment on individual cases.

Thursday's agreement could net Germany billions of euros in tax revenues from individuals who have stashed savings in Swiss accounts to avoid tax.

The governments of both countries had hoped the deal would end a diplomatic spat that has dragged on for years.

But Germany's opposition Social Democrats have threatened to scupper the legislation, saying they will vote against it in the upper house, where Chancellor Angela Merkel's centre-right coalition is short of a majority.

The two governments had to revise the deal to make the terms tougher after the SPD rejected a previous version.

The SPD says the law will take effect too late, in 2013, and that it gives time to tax evaders to move their savings and remain anonymous.

Merkel's government is betting that SPD-led states will ultimately drop their objections and support the deal as it will bring them a huge windfall.

North Rhine-Westphalia, which has an SPD-led minority government, holds regional elections next month in which an alliance of the centre-left party and the Greens is tipped to win a majority.

Germans hold an estimated 150 billion Swiss francs in Swiss bank accounts.

(Writing by Madeline Chambers; Editing by John Stonestreet)