The Dutch tax office has for three years been tracking payments made by Dutch people in the Netherlands using foreign bank cards in an effort to trace illicit foreign savings accounts, the NRC reports on Thursday.

All payments made using foreign cards are tracked but those made by tourists are filtered out, leaving officials with hundreds of millions of Dutch payments to process, Hans van der Vlist, director of the Fiod tax investigation service, told the paper.

Details of spending carried out via around one thousand cards have now been handed over to the Fiod investigators for further checking.

In some cases people who have told the tax office they don’t have any assets abroad have been caught out by making major purchases with foreign bank cards. ‘They’ve bought a car or an expensive watch or withdrawn a lot of cash,’ Van der Vlist said. ‘We don’t yet know what all those accounts include.’

The investigations are expected to take at least until the end of the year.