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China's biggest microblogging service has introduced a code of conduct explicitly restricting the type of messages that can be posted.

Sina Weibo - which resembles Twitter - took the action after local authorities criticised "unfounded" rumours posted by some users.

Reports suggest a credit score system will also be introduced with points deducted for rule breaches.

Repeat offenders face having their accounts deleted.

The service's parent, Sina Corp, says it has more than 300 million registered users.

Users are reported to start with 80 points - they gain more by taking part in promotional activities, but lose points if they break any of the rules.

It is reported that if a subscriber's points fell below 60 a "low credit" warning would appear on their microblog, leading to the possible cancellation of their account if it hit zero. If they "behaved" for two consecutive months their score is reported to return to 80.

"This is a sign of the authorities trying to restrain the internet in China, but a hardcore group of people will still find ways to get round the restraints," Dr Kerry Brown, head of the Asia Programme at the Chatham House think tank, told the 91�ȱ�.

"There is a tradition of indirect criticism in which people make points using coded references. I very much doubt these rules will change anything."

Restricted speech

The news was first reported in the western press <link> <caption>by The Next Web</caption> <url href="http://thenextweb.com/asia/2012/05/09/sina-weibo-to-introduce-user-contract-on-may-28-as-chinas-microblog-crackdown-continues/" platform="highweb"/> </link> which quoted from a <link> <caption>translated version of the rules</caption> <url href="https://docs.google.com/document/d/1WG7TfsQLE6DiCSO2PCaSAdZc19xiojGRyZ2ORgU1oPE/edit?pli=1" platform="highweb"/> </link> created by an anonymous group of volunteers.

The "community convention" says its members may not use the service to:

It adds that members must not use "oblique expressions or other methods" to circumvent the rules.

Users have sometimes abbreviated names or used code words to avoid detection in the past.

Rumours

<link> <caption>The Tech in Asia blog</caption> <url href="http://www.techinasia.com/sina-weibos-user-contract-big-deal/" platform="highweb"/> </link> noted that Sina did not invent the rules.

"They are pulled directly from Chinese law and are applicable to Weibo posts regardless of whether Sina includes them in a user contract or not," it said.

However, it added that Sina's credit score system was an innovation.

A committee made up of experts and Sina Weibo subscribers will be charged with enforcing the rules.

Sina - and its competitors Baidu and Tencent - were ordered to ensure all their members registered their real identities by March. However, Sina later admitted it had not fully implemented the order.

Last month Chinese officials forced Sina and Tencent to suspend users' ability to comment on each other's posts for three days after allowing rumours to spread.

The official news agency, Xinhua, reported that the sites <link> <caption>"pledged to strengthen management"</caption> <url href="http://news.xinhuanet.com/english/china/2012-03/31/c_122911627.htm" platform="highweb"/> </link> afterwards.

Authorities have been critical of false reports spread through microblogs including news of the assassination of North Korean leader Kim Jong-un, and stories of a military coup that tried to overthrow Chinese President Hu Jintao.

Chinese social media has also been pressured to filter posts featuring words associated with controversial events.

When the former Communist Party's Chongqing chief, Bo Xilai, was stripped of his Politburo post several sites would not deliver results for searches featuring his name.