UK operator O2 has apologised for leaking some of its customers’ mobile number details to website owners.

The company, which is part of Spain’s Telefónica group, explained that it is standard practice for it to provide customers’ mobile numbers to certain website owners when they browse their sites, but this agreement only exists between O2 and its trusted partners.

The company explained that it shares mobile numbers with these partners for three reasons: to manage age verifications for adult content, to enable third party companies to bill for premium content such as ringtones and to identify customers using O2 services, such as My O2 and Priority Moments.

However, an error at the operator caused those details to be sent to a wider spread of website owners that are not O2’s trusted partners. This was caused by technical changes that the operator implemented as part of routine maintenance, but which mistakenly allowed website owners to see the mobile numbers of those browsing their site.

According to O2, this happened between January 10, 2012 and January 25, 2012 and the issue has now been fixed.

The company said that the only information that it divulged was its customer phone numbers, and no other personal details, and these will only have been leaked if the user had accessed websites that are not trusted partners, using 3G or WAP services.

The UK Information Commissioner’s Office (ICO) is investigation the blunder and O2 admitted it is co-operating fully with the ICO, and has also contacted UK regulator Ofcom.

“Security is of the utmost importance to us and we take the protection of our customers’ data extremely seriously,” read an O2 statement. “We would like to apologise for the concern we have caused.”

While some O2 customers praised the firm’s quick response in online forums, others demanded to be given a list of the “trusted partners” that were allowed to receive phone number details, and requested that they be able to opt out of the practice.