Search engine Yahoo has confirmed that its websites hosted malware-spreading malicious adverts on its European websites.

Users in the UK, France and Romania had been the worst affected by malicious ads inserted by hackers into the ads.yahoo.com advert system.

"On Friday, January 3 on our European sites, we served some advertisements that did not meet our editorial guidelines, specifically they spread malware," Yahoo said in a statement.

It was estimated that 27,000 users could have been infected by malware per hour.