Switzerland’s reputation as a safe haven for undeclared wealth was in tatters last night after German authorities said that they were investigating more than 1,000 customers of Crédit Suisse for alleged tax evasion.

The inquiry by prosecutors in Düsseldorf comes after officials in the German state of North Rhine-Westphalia acquired the names of the Swiss bank’s account-holders on a disc that was thought to have been stolen. German reports said that the state paid €2.5 million (£2.2 million) for the information.

The move was part of a sweeping campaign against tax evasion by the German authorities, who said publicly that they were prepared to pay informants for data on Germans with money in Swiss bank accounts. Switzerland reacted furiously to the charge, which it regards as a further attack on its image.

Doris Leuthard, the Swiss President, accused Germany of a creating a market for secret data and encouraging Swiss bank employees to steal confidential information.

Anger in Berne is certain to be inflamed by the launch of a formal inquiry. The prosecution service in Düsseldorf said that it was investigating 1,100 Crédit Suisse customers for alleged tax evasion, but also Credit Suisse staff on suspicion of complicity.

Dirk Negenborn, a spokesman for the prosecutor’s office, said that a total of about €1.2 billion was held in the accounts under investigation.

Despite Swiss protests, German officials have suggested that they may make the stolen data available to tax inspectors in other countries.

A Crédit Suisse spokesman said that the bank could not comment.

The campaign marks a shift in Berlin’s approach towards Germans holding funds in Swiss bank accounts that have not been declared to the tax authorities. They have become the focus of official anger as the economic crisis reshapes political doctrines and forces the Government to seek additional tax revenue.

About 3,000 evaders — with accounts in Germany as well as abroad — have come forward to make spontaneous declarations since the campaign began earlier this year, according to officials. Under German law, they can avoid prosecution if they pay what they owe. One estimate suggested that they had paid a total of €500 million to the authorities — a sum that could double by the end of the year.

However, Swiss banks are reeling from the latest assault on their traditional secrecy, which comes less than a month after HSBC revealed that information on 24,000 accounts at its Geneva unit had been stolen by a computer technician.

Hervé Falciani, the former employee, made the data available to French prosecutors, who are using it in an attempt to trace Gallic tax evaders with HSBC accounts in Geneva.

Paris said that it was hoping to pinpoint thousands of French evaders as a result of the inquiry.

Mr Falciani reportedly sought to make the information available to several governments, including Britain’s, before it was obtained by France, where he is now living.

Last year, the Swiss Government settled a long-running dispute with tax authorities in the United States by agreeing to hand over details of 4,450 accounts held by Americans who allegedly had evaded taxes.