Update: O2 claims to have fixed the issue -- and indeed, two O2 phones we've tested are no longer displaying their numbers on the test site. Read more about O2's response here. Original story follows:
O2 is in boiling hot water today -- it looks as though the mobile network has been sending your mobile number to every site you visit.
The issue was spotted by one Lewis Peckover, who set up a site that displays all the information he can glean from you when you visit. Go to his test site from a phone connected over O2's data network, and there's every chance your mobile number will pop up in the list marked 'Headers received'.
No need to check for yourself (you would be giving the site your phone number after all) -- we've tested the site using two O2 SIMs and found that the phone number did indeed pop up on both.
We tested the site from phones on other networks. T-Mobile didn't seem to divulge the number of the phone we were testing, neither did Orange. Vodafone also came up clean on the two SIMs we tested.
Tesco Mobile and GiffGaff are affected, however, at least on the SIMs we tested, because they both piggyback on O2's network.
We've spotted reports from people saying their O2 phones were not affected, so it's possible that not every mobile will hand over your digits.
Peckover's testing site appears to have gone down very recently, probably buckling under the weight of irritated O2 customers checking to see if their mobile number pops up.
This is bad, because it means any site you visit from a phone on O2's data network could be grabbing your mobile number. That's obviously a serious privacy hiccup.
We've contacted O2 asking for comment, and the company has told us it's investigating the issue. We'll let you know as soon as we have more information. This is a developing story, and we'll be keeping a close eye on what happens next.
In the meantime, let us know what you think in the comments or on our Facebook wall.