More than 700 pages of confidential information from Europol have ended up in the hands of Dutch tv current affairs Zembla in what the European police agency says is ‘an extremely serious incident’.

The documents, which mainly concern the period 2006 to 2008, contain hundreds of names and phone number of people who are thought to have links to terrorism. They include 54 different investigations, including analyses of the Madrid railway bombings and the Dutch ‘Hofstadgroep’ anti-terror probe.

The documents are thought to have reached Zembla after a Europol worker took them home, which is against the rules. She copied them to a private back-up network known as iOmega which was linked to the internet without a secure connection. Zembla says it ‘found’ the files on the back-up disc, but does not say how it was alerted to their presence.

‘This damages confidentiality and is the reason why we have immediately launched an investigation to find out how it happened,’ Europol deputy director Wil van Gemert told Zembla.

The leak does not impact current investigations and there are no indications that any organisation apart from Zembla has had access to the documents, he said.

The Europol staffer had worked for the agency for 11 years and is now working for the Dutch police.

The Zembla programme about the files will be broadcast on Wednesday evening