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, as he escalates an extraordinary war of words against his church.

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, a leading critic of the Vatican's outreach to China, 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. Zen doubled down on Monday and denounced church officials in Rome for betraying Chinese worshippers in what amounted to a highly unusual attack from a clergyman against the Holy See.

"Mainland brothers and sisters fear not losing all they have, the prison cell or shedding their blood," Zen wrote. "Their greatest suffering is being sold out by their 'loved ones.'"

The proposed changing of the bishops was the clearest evidence yet of the Vatican's effort to reach a deal with China, a country with an estimated 12 million Catholics. Of those, about half worship in "underground" churches that recognize only Rome as their highest authority while the rest belong to state-authorized churches with clergy named by Beijing.

"Priests and congregants will have many long nights of suffering over the prospect of obeying and respecting those priests who were illegitimate today but will be legitimized by the Holy See tomorrow, having been approved by the government," Zen wrote.

The Vatican had no immediate comment.