The Open Rights Group has complained that some UK mobile operators are currently blocking access to an internet browsing anonymiser service called Tor.
Tor allows people to connect to websites on the internet with some degree of anonymity, essentially providing an IP routing service to users, in order to help remove identifiable information by passing through a number of IP 'hops'.
In some cases, it has been used by people living under dictatorial regimes that do not allow free access to the wider internet to use blocked services like Facebook and Twitter.
However, Vodafone, Three and O2 have all been blocking access to Tor's main website without informing users clearly of how to remove the blocks, the Open Rights Group (ORG) said on Monday. The blocks generally only apply to pre-pay customers that have not provided credit card details.
"Phone companies 'filter' the mobile internet because they don't know whether their phones are being given to or used by children and young adults. This happens by default in most cases," Peter Bradwell, a campaigner at the ORG said in a blog post.
"These problems would be less of an issue if it was clear to users when filtering applied to their account, how to turn the filter off, and how to report mistakes. But this isn't the case," he added.
O2 told ZDNet UK on Tuesday that the filtering is deliberate and is done as a precautionary measure to protect children from accessing adult content.
"Any site classified as an 'anonymiser' site cannot be accessed by children due to the protection mechanism we have in place to stop them from accessing adult content," an O2 spokesman told ZDNet UK. "This is because anonymiser sites bypass the adult content filters we have in place and thus could enable children to access unrestricted material."
The spokesman added that access to blocked sites is allowed as long as the user is over 18 and has gone through the age-verification process.
In contrast to O2, a Vodafone spokeswoman told ZDNet UK that the blocking of the main Tor website was a mistake and that it would be rectified.
"[Tor] has been blocked by our adult content filter. It shouldn't be blocked and we will be offering access to it again shortly," the spokeswoman said.
Three UK and T-Mobile had not responded to a request for comment at the time of writing.