Chinese officials are banned from religious activities even after they retire, state media said on Friday.

Reuters reports that Communist Party officials must oppose what the government calls "cults" and not become religious, even after they have left state service, according to new rules issued from the party's powerful Organisation Department.

Although the Chinese constitution allows for freedom of belief, in reality the officially-atheist ruling Communist Party keeps a tight rein on religious activities. Officials who are party members are not supposed to practise religion.

The Organisation Department, which oversees personnel decisions, has now said retired officials who were also party members have to follow these rules too.

"There are clear rules that retired officials who are party members cannot believe in religion and cannot attend religious activities, and must resolutely struggle against evil cults," an unnamed official said in the lengthy explanation.

Retired officials must also be aware that "certain ethnic minority folk custom activities" they happen to attend are nothing to do with religion, the official added, without elaborating.

The new rules continue a crackdown on religious activity in China with a number of pastors and human rights activists detained in recent months. Among them is Pastor Gu Yuese of China's largest Protestant church, the 10,000-member Chongyi Church, who was removed from his position without warning by the authorities, accused of embezzling church funds and detained in a so-called 'black jail'.

His treatment has been linked by human rights activists with his stand against the removal of crosses from churches. Demonstrations against this programme have been crushed with force and numerous church buildings have been demolished.

The growth of Christianity in China has been exponential and authorities have targeted churches, which they see as dangerous cults.

Additional reporting from Reuters.