A former archbishop accused of child sex offences was has died a month after he was hospitalised on the eve of his trial on paedophile charges, the Vatican said.

Jozef Wesolowski had been charged with possessing child pornography in Rome in 2013-14, and the sexual abuse of minors during his 2008-13 stint as the Vatican ambassador in the Dominican Republic.

A former papal ambassador to the Dominican Republic, Wesolowski appeared to have died of natural causes, the Vatican said in a statement.

The 67-year-old Pole was found dead in his rooms in the papal state, where he had been under arrest since September.

Wesolowski fell ill in July and was taken to hospital the day before his trial, seen as an important test of Pope Francis's drive to clean up the Catholic Church after child sex abuse scandals in several countries over many years.

He served as Vatican ambassador in Santo Domingo for five years.

He was recalled to Rome by the Vatican in 2013 and stripped of his duties after Dominican media said he paid boys to perform sexual acts, leading to a police investigation.

The Vatican rarely defrocks archbishops, indicating that it took seriously the accusations against him.

According to Dominican media, Wesolowski would dress in layman's clothes, don a baseball cap and visit a beachfront area known to be frequented by impoverished child prostitutes.

The charge sheet for his trial accused him of sexually abusing an undisclosed number of boys "presumably" aged between 13 and 16, saying the abuse took place in public on least one occasion.

The pope has promised zero tolerance of sex abuse of minors and had rewritten Church rules to allow the trial to take place, but victims groups say he has not done enough.

The Vatican said the results of the postmortem examination would be made public as soon as possible.