Google says there is no evidence its systems have been hacked after almost five million Gmail addresses and passwords were posted online.

Experts believe the combinations are either old ones obtained through phishing attacks, or passwords used on other sites in conjunction with Gmail addresses.

The company claims only 2% would have actually worked and that affected users have been told.

Some estimates put the number of Gmail users at 425 million people.

Gmail was launched on April Fools' Day in 2004 and is owned by the search engine giant, Google.