China has been monitoring and censoring messages sent through the Skype internet service, researchers have found.

Citizen Lab, a Canadian research group, says it has uncovered a database that contains thousands of politically sensitive words which have been blocked by China. The publicly available database also revealed personal data on subscribers to the service.

Skype said it has always been open about the filtering of data by Chinese partners, but that it was concerned by breaches in the sites security. The researchers at Citizen Lab, based at the University of Toronto, said they discovered a large surveillance system which has picked up and stored messages sent through the online telephone and text messaging service.

The database reportedly held more than 150,000 messages that included words such as “democracy” and “Tibet”, along with phrases related to the banned spiritual movement, Falum Gong.

Citizen Lab’s report, entitled “Breaching Trust”, says that, “These text messages, along with millions of records containing personal information, are stored on insecure publicly accessible web servers.”

The researchers said that the by using just one username, it was entirely possible to identify all the people who had received or sent messages from the original user.

Skype is known as Tom-Skype in China, and is a joint venture involving the American auction site eBay and Chinese company TOM-Online. Citizen Lab said it was “clear” that Tom was “engaging in extensive surveillance with seemingly little regard for the security and privacy of Skype users”.

The report asked to what extent Tom Online and Skype were co-operating with the Chinese government in monitoring communications, but Skype president Josh Silverman said China’s monitoring was “common knowledge” and that Tom-Online had “established procedures to meet local laws and regulations”.

“These regulations include the requirement to monitor and block instant messages containing certain words deemed offensive by the Chinese authorities,” he said.

Mr Silverman said that it had been Tom-Online’s policy to block certain messages, and then delete them. He has sated that he would be investigating why the policy changed to allow the company to upload and store those messages.

Although China has one of the highest internet using populations, the authorities have long-prevented citizens which are considered politically sensitive. Large online companies have been criticised for working with China’s strict regulations, with Google, Microsoft and Yahoo, all guilty of doing so.