We’ve got some alarming news for UK O2 subscribers, it appears that the carrier is sending your mobile number in plain text to every website you visit via mobile internet.

The issue was discovered by Lewis Peckover, who built a simple website page to check and display the header information sent by mobile browsers when visiting a website.  The usual stuff is all there, Host, User Agent Referrer and so on, but there is a worrying extra field-  X-up-calling-ine-id– In other words, your mobile phone number.

We tested it ourselves with an O2-connected iPhone and got the same results:

Angry O2  customers have bombarded the company’s Twitter page with complaints, and they do seem to be aware of the problem.  O2 responded with:

“Security is our top most priority, we’re investigating this at the moment & will come back with more info as soon as we can.”

Orange, T-Mobile and Vodafone numbers don’t seem to be affected by this kind of issue, but Tesco Mobile and GiffGaff  (they operate on the same network) are .