Floodwaters drenched most of Venice on Wednesday, reaching a record 143 centimetres (56 inches) for the year, authorities said.

Some 56 percent of Venice was under the water, caused by wind and rain combined with the lagoon city's periodic tidal phenomenon, a monitoring and forecasting centre said.

The phenomon known simply as "acqua alta" (high water) was highest at around 5:00 am (0400 GMT), the centre said.

As usual, workers placed elevated wooden platforms on main passageways in the Renaissance city.

The monitoring centre said the floodwaters were unlikely to recede before Friday, Christmas Day, in a city also swamped by thousands of holiday revellers.

Venice suffered its worst "acqua alta" on November 4, 1966, when the city was submerged by 1.94 metres of water amid catastrophic flooding throughout Italy.

The city has for years been wrestling with the problems posed by the threat of rising sea levels.