The volcanic eruption that destroyed the ancient Roman city of Pompeii probably took place two months later than previously thought, Italian officials said on Tuesday.

Historians have traditionally dated the disaster to August 24, 79 AD, but excavations on the vast site in southern Italy have unearthed a charcoal inscription written on a wall that includes a date which corresponds to October 17.

The writing came from an area in a house that was apparently being renovated just before the nearby Mount Vesuvius erupted, burying Pompeii under a thick blanket of ash and rock.

"Being charcoal, fragile and evanescent, which could not last a long time, it is more than likely that it was written in October 79 AD," said Massimo Osanna, head of the Pompeii site.

The August 24 date derives from an account of the blast given by Pliny the Younger, who witnessed the eruption and wrote about it almost 30 years after the event in two letters to his friend, the Roman historian Tacitus.

However, previous excavations have uncovered a calcified branch bearing berries that normally only come out in autumn. The discovery of some braziers over the years also suggested the disaster did not strike at the height of summer.

Osanna suggested the correct date might have been October 24.

Showing off the faint writing on an uncovered white wall, Culture Minister Alberto Bonisoli hailed it as an "extraordinary discovery".

"Today, with a lot of humility, maybe we're rewriting the history books because we're dating the eruption to the second half of October," Bonisoli said.