Progress in the relationship has been bogged down by issues such as Beijing's unilateral ordination of bishops and Rome's diplomatic ties with Taiwan.

Liu Bainian , a deputy chairman of the Chinese Catholic Patriotic Association, said the group had not been officially notified of the letter's release.

More than 100 bishops from government-sanctioned churches around the mainland ended a two-day meeting in the Beijing suburb of Huairou yesterday.

Zhan Silu , another patriotic association deputy chairman and bishop of the Mindong diocese in Fujian , said the meeting was held to discuss celebration activities for the 50th anniversary of the association, the official body that oversees church affairs on the mainland.

But some church sources said the event was held partly in response to the letter.

'This meeting had not been planned,' a Catholic priest in Beijing said. 'The bishops were reminded to toe the government line [after the letter comes out].'

Both Bishop Zhan and Mr Liu denied the letter was discussed at the meeting. 'We don't even know when the letter is coming out and what it is about, so how could we discuss it?' Mr Liu said.

The United Front Work Department organised the meeting.

Underground bishops, who are only approved by the Holy See, but not the patriotic association, were not invited.

The letter is expected to highlight the unity between the unregistered, or so-called underground church and the official church. It is also expected to underline the Catholic Church's power to appoint bishops on the mainland.

Beijing started to ordain its own bishops without Rome's consent in 1958, a move that has been interpreted as a reluctance by the communist government to cede authority and control over the Catholic Church on the mainland.

The patriotic association claims to be autonomous from the Pope and this assertion has been criticised as a deviation from Catholicism, prompting church-goers loyal to the Pope and who regard the association as unauthorised to turn to underground churches.