The church where the tradition of celebrating Christmas on Dec. 25 may have begun was built near a pagan shrine as part of an effort to spread Christianity, a leading Italian scholar says.

Italian archeologists last month unveiled an underground grotto that they believe ancient Romans revered as the place where a wolf nursed Rome's legendary founder Romulus and his twin brother Remus.

A few metres from the grotto, or "Lupercale," the Emperor Constantine built the Basilica of St. Anastasia, where some believe Christmas was first celebrated on Dec. 25.

Constantine ended the frequent waves of anti-Christian persecutions in the Roman empire by making Christianity a lawful religion in 313. He played a key role in unifying the beliefs and practices of the early followers of Jesus.

In 325, he convened the Council of Nicaea, which fixed the dates of important Christian festivals. It opted to mark Christmas, then celebrated at varying dates, on Dec. 25 to coincide with the Roman festival celebrating the birth of the sun god, Andrea Carandini, a professor of archeology at Rome's La Sapienza University, told reporters Friday.

The Basilica of St. Anastasia was built as soon as a year after the Nicaean Council. It probably was where Christmas was first marked on Dec. 25, part of broader efforts to link pagan practices to Christian celebrations in the early days of the new religion, Carandini said.

"The church was built to Christianize these pagan places of worship," he said. "It was normal to put a church near these places to try to 'save' them."

Rome's archeological superintendent Angelo Bottini, who did not take part in Carandini's research, said that hypothesis was "evocative and coherent" and "helps us understand the mechanisms of the passage from paganism to Christianity."

Bottini and Carandini both said future digs could bolster the link between the shrine and the church if structures belonging to the "Lupercale" are found directly below the basilica.

The Basilica St. Anastasia was the first church to rise not on the ancient city's outskirts, but on the Palatine Hill, the palatial centre of power and religion in imperial Rome, Carandini said. Though little known today, at the time of Constantine it was one of the most important basilicas for Christians in Rome, he said.

The "Lupercale" shrine - named after the "lupa," Latin for she-wolf - is 15 metres below ground. So far, archeologists have only been able to see it by inserting probes and cameras that have revealed a vaulted ceiling decorated with coloured marble and a white imperial eagle.

Though some experts have expressed doubts that the grotto is in fact the mythological nursery of Romulus and Remus, most archeologists believe the shrine fits the descriptions found in ancient texts, and plans are being drawn up to excavate the structure further.