The tax office was wrong to fine people for keeping their savings secret based on information bought from an anonymous source, appeal court judges in Den Bosch said on Tuesday.

The court said that the tax office should not have used information provided by an unnamed source, who sold officials details about dozens of secret accounts in Luxembourg in 2009.

The case was brought by relatives of one of the people given a heavy fine for keeping savings hidden from the tax office.

The court said that person who made the anonymous tip off is likely to have obtained the information through criminal means. While that is not illegal in itself, the government should have made a more balanced decision about what to do with the information and explained its actions to the court.

In addition, the tax office had failed to establish how the person making the tip off had obtained the information and if he had a criminal past. Nor is the tax office willing to say how much he had been paid for the information, the court said.

Judges had earlier ordered the tax office to come clean about the payment but officials have consistently refused to do so.