Mobile phone numbers of customers surfing the web via O2 have been leaked to every site they visited in the last two weeks.
The network operator, which has 24 million UK customers, has apologised for the technical glitch, which saw the numbers included in information sent to websites articulating how they should display on mobile phones using O2.
The Information Commissioner's Office has confirmed it will investigate. But a spokesman said there were no grounds to suggest the issue spread beyond O2.
An O2 spokesman said the issue had now been resolved.