O2 is facing a backlash from its customers after it was claimed the firm is passing users' mobile phone numbers to any web site they visit when accessing the internet over a data connection.
An online report shows that visiting a site causes the user's number to be displayed, alongside other information on the device accessing that site such as its operating system and model number.
V3 tested the claim on an HTC Mozart device and can confirm the device's phone number was displayed.
Tests with phones on Orange, Vodafone and T-Mobile did not display any numbers, suggesting it is only O2 that provides this information.
V3 contacted O2 for comment on the issue and the firm said it was investigating the reports, but had issued no statement as yet.
Meanwhile the Information Commissioner's Office (ICO) said it was aware of the claims and was planning to contact O2 to discuss the incident.
"When people visit a web site via their mobile phone they would not expect their number to be made available to that web site," it said.
"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."
Security expert Graham Cluley from Sophos pointed out that the discovery of such a measure is not actually new, but said that it was strange O2 would be engaging in such a practice.
"It's hard to understand why a mobile phone network operator would think it is necessary to transmit their customers' mobile phone numbers to the web site they visit," he said in a blog post.
"My guess is that it's more likely to be a cock-up than malice which caused this data to be leaked - but what's worse is that the problem is still present almost two years after it was first discovered."
Update at 14:54: The issue appears to be fixed now, with the device's number no longer showing on the test page. The firm has also issued a blog claiming it has fixed the issue and apologised to customers.
Acton's warnings come as Facebook is embroiled in one of the biggest data scandals in history
The unmanned tanks could eventually be kitted with AI systems
Dubbed I-MacEtch, it will help meet demand for more powerful nano-tech
GPU firm's research unit for self-driving cars is growing