The Vatican has received a ransom demand in return for two letters written by Michelangelo, a spokesperson has confirmed.

Father Federico Lombardi said a cardinal in charge of St Peter's Basilica had been offered a sum of money in exchange for the letters, which went missing from Vatican archives in 1997. The offer was "naturally refused because these are stolen documents," Lombardi said.

According to Rome newspaper Il Messaggero, the demand for €100,000 (£72,000) was made by a former Vatican employee. The newspaper only mentioned one letter, however, which was apparently written entirely by Michelangelo, and features his signature.

The theft of the documents was not made public until Sunday, despite having occurred nearly two decades ago. Lombardi said the discovery was originally made by a nun. He confirmed that Vatican police were investigating the case, but gave no further details about the letters themselves.

According to The Times, the signed letter is believed to include information about art commissioned by the Pope.

Michelangelo famously painted the ceiling of the Sistine Chapel in the 16th century, as well as The Last Judgement – a freso on the altar wall of the chapel. He was appointed architect for St Peter's Basilica in 1546, and was a principal designer of the building which remains one of the largest churches in the world.