The letter, due to appear within weeks, will mark a big step in the Holy See's effort to open official ties with China, severed after the Communist Party came to power in 1949.

But it would focus on forging unity among believers split between China's state-backed church and an "underground" church loyal to Rome, said the source familiar with dealings between the Vatican and Beijing.

"His primary concern is to help the reconciliation between the two communities," said the Hong Kong-based church source, speaking by phone overnight. He requested anonymity, citing the sensitivity of ties.

"I think there will be a strong invitation to the two sides to try to work together to heal those differences."

Beijing and the Vatican have been estranged since the Chinese Communist Party came to power. The party first persecuted the faithful and then in the past three decades allowed worship only in state-controlled churches, mosques and temples.

In recent years, the two sides have edged towards restoring formal ties, and a Vatican meeting in January focused on China. Since his election in April 2005, Pope Benedict has made normalizing relations one of his priorities.

The Vatican is the only European state to keep diplomatic ties with Taiwan, the self-governed island that Beijing says is a breakaway province that must accept reunification.

The Pope's planned letter, however, highlights how the future of the Chinese church rests not just on diplomatic negotiations but on relations among millions of believers who share basic beliefs but have been split by politics and history.

China's 10 million or more Catholics are divided between the government-approved church, which honors the Pope but accepts controls demanded by the Communist Party, and the underground church which recognizes only Vatican authority.

In recent years, that divide has created tensions over the appointment of bishops, with Chinese authorities naming some bishops who have not won the blessing of the Pope, something even clergy in the state-backed church now often seek and get.

The most prominent bishop in the state-backed church, Fu Tieshan, died last week, opening a vacancy in the sensitive Beijing diocese. Fu, who also used the Christian name Michael, did not have the Vatican's blessing.

The church source said the Holy See had hopes that Fu's successor would have papal approval. Beijing authorities appeared divided over whether Fu's successor should have Vatican blessing.

"They have to resolve their internal issues," he said.

The vice chairman of the state-controlled Chinese Catholic Patriotic Association, Liu Bainian, who often speaks on behalf of the state church, was notably tight-lipped when asked about Vatican involvement in choosing Fu's successor.

"Now we're attending to Bishop Fu's passing and aren't concerned about those issues," he said.

Asked about the Pope's role in choosing bishops, Chinese Foreign Ministry spokesman Liu Jianchao said the Vatican should not "interfere in China's internal affairs". But Liu also repeated that China hoped for better ties.

The Pope's letter would spell out his position that bishops must come under his authority, and would set forth broader demands for religious freedom, the church source said.

"It's almost impossible for the bishops to meet freely on their own. There are always officials around," he said.

Pope Benedict would send his letter to China's foreign ministry days before its public release "as a friendly gesture", he said.