O2 says it has now fixed a data breach which saw mobile internet users' phone numbers given out to websites.
The mobile giant admits a technical glitch meant customers' numbers were sent to website owners when O2 users visited sites on their handsets.
O2 says it is normal for it to disclose numbers to sites with premium or adult content, or that are part of its Priority Moments discount service.
But the firm has apologised after users' numbers were also revealed to all manner of other sites between 10 January and 2pm yesterday.
When MoneySavingExpert.com checked yesterday, we found mobile numbers were disclosed on a site specially designed to notify users of what data it receives on users. When we checked this morning, numbers no longer appeared.
An O2 spokesman says: "There has been the potential for disclosure of customers' mobile phone numbers to further website owners."
O2 users on our forum fear the breach could lead to a surge of spam calls.
'Keeping information secure'
One user, drewziph, says: "I would have thought that not protecting customers data was both illegal due to the Data Protection Act and a violation of the contract terms and conditions.
"I've been affected from spam messages and calls today and am monitoring what to do next."
The Information Commissioner's Office (ICO), which regulates the protection of personal data, is keeping its options open over whether to take action.
A spokesperson says: "Keeping people's personal information secure is a fundamental principle that sits at the heart of the Data Protection Act and the Privacy and Electronic Communications Regulations.
"When people visit a website via their mobile phone they would not expect their number to be made available to that website.
"We will now speak to O2 to remind them of their data breach notification obligations, and to better understand what has happened, before we decide how to proceed."
An O2 spokesman adds: "Security is of the utmost importance to us and we take the protection of our customers' data extremely seriously.
"We would like to apologise for the concern we have caused."