The retired archbishop of Hong Kong has slammed the Holy See's negotiations with the Chinese government as a "catastrophe" that would bring suffering to millions of worshippers.

Retired Cardinal Joseph Zen warned in a blog post this week that some Chinese Catholics who follow so-called underground churches are at risk of arrest even while the Catholic Church pushes for a historic breakthrough in relations with China's ruling Communist Party.

Zen is a leading critic of the Vatican's outreach to China. He revealed in an unusual statement last month that the Vatican had asked a legitimate "underground" bishop to stand down in favor of an excommunicated one favored by Beijing.

The proposed changing of the guard was the clearest evidence yet of the Vatican's effort to reach a deal with China.