Passing information on financial transactions from around the world to the CIA may have been a breach of UK and European law, the information commissioner has told a newspaper.

According to a report published in The Guardian newspaper, the information commissioner, which is responsible for enforcing the Data Protection Act, is investigating the covert operation which saw the Belgian-based Society for Worldwide Interbank Financial Telecommunication (Swift) hand over international financial records to the US agency. The records include the details of an estimated 4.6 million British banking transactions.

The classified program, which began shortly after the September 11 attacks, tracks the financial transactions of people suspected of having ties to terrorist networks. A spokesman for the commissioner told the Guardian that the privacy issue was being taken very seriously and is investigating the matter.

Swift responded to media reports in June when concerns were raised in the US over the program. The organization said in a statement: Our fundamental principle has been to preserve the confidentiality of our users’ data while complying with the lawful obligations in countries where we operate.

The report also said that the UK government was fully aware of the program but had no reason to believe that the operation was unlawful, although a spokesperson would not comment on the investigation by the commissioner.