The website of London anti-piracy firm ACS:Law has been attacked, leading to the leak of email archives and personal data of thousands of internet users.

It is understood that the names and addresses of more than 5,000 people alleged by the firm to have engaged in illegal file-sharing were posted on the internet following an attack on the firm’s website.

ACS:Law acts on behalf of clients who believe their copyright in music, software, computer games and films has been infringed by illegal file-sharers. Reports have suggested that the attack on the firms’ website was made in retaliation for its anti-piracy work.

A spokesman for the Information Commissioner’s Office said Andrew Crossley, principal at ACS:Law, had reported the breach himself.

He added: ‘The ICO will be contacting ACS:Law to establish further facts of the case and to identify what action, if any, needs to be taken.’

Crossley told the Gazette he could not go into the details, but said: ‘Last Friday (24 September) my firm’s external website and email host was illegally hacked following a coordinated attack. Emails were accessed and subsequently placed on the internet, as a result of unlawful and criminal activity.

‘I have notified the police and the ICO, and I have been in communication with the Solicitors Regulation Authority. My investigations into what has happened are ongoing.’

A SRA spokesman said the regulator was aware of the leak and is investigating.