Venice is bracing for an unprecedented third major flooding in less than a week, with sea water due to swamp the already devastated historic city today, where authorities have declared a state of emergency.

Mayor Luigi Brugnaro ordered the iconic St Mark's Square closed on Friday as the latest sea surge struck with strong storms and winds battering the region.

After a brief respite yesterday, the city forecast a high water of 160cm for just after midday today, lower than Tuesday's high of 187cm but still dangerous.

Churches, shops and homes in the city, a UNESCO World Heritage site, have been inundated by unusually intense "acqua alta", or high water, which on Tuesday hit its highest level in half a century.

"We've destroyed Venice, we're talking about €1bn in damage," Mr Brugnaro said after the second major flooding of around 160cm hit on Friday.

The Italian football team travelled to Venice yesterday to show solidarity with the stricken city.

"On behalf of the whole team, we stand close to the city of Venice," said Italy goalkeeper Gianluigi Donnarumma.

The Italian Football Federation delegation visited several businesses damaged by the flooding, and chatted with Venetians, volunteers and police.

Gabriele Gravina (L) and Italy's goalkeeper Gianluigi Donnarumma (R) in St Mark's Square, Venice

"Venice will overcome this too. Like an athlete who suffers a serious injury and then gets up again," said delegation chief and former Azzurri international Gianluca Vialli.

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The crisis has prompted the government to release €20 million in funds to tackle the devastation.

Surveying the damage, Culture Minister Dario Franceschini warned the task of repairing the city would be huge. More than 50 churches had suffered damage, he said.

Residents whose houses have been hit are eligible for up to €5,000 in immediate government aid, while restaurant and shop owners can receive up to €20,000 and apply for more later.

Most of the city's cash machines were no longer working because of the water, making life even more difficult for tourists and Venetians.

Despite being used to the inconvenience of their city's rising waters, some inhabitants expressed frustration.

"All the stock in the basement is lost," lamented Mr Luciano, a worker at a shop along St Mark's Square.

He said he remembered well the infamous "acqua alta" of 1966, when the water rose to a level of 1.94 metres, the highest since records began in 1923.

"These so frequent high waters have never happened before... this time there's so much more damage than in the past," he said.

Hotels reported cancelled reservations, some as far ahead as December, following the widespread diffusion of images of Venice underwater.

Tuesday's high waters submerged around 80% of the city, officials said.

Many, including Venice's mayor, have blamed the disaster on global warming and warned that the country prone to natural disasters must wake up to the risks posed by ever more volatile seasons.

The Serenissima, as the floating city is called, is home to 50,000 residents but receives 36 million visitors each year.

A massive infrastructure project called MOSE has been under way since 2003 to protect the city, but it has been plagued by cost overruns, corruption scandals and delays.

The speaker of Italy's Senate, Maria Elisabetta Alberti Casellati, visited Venice yesterday and called for the MOSE project to be completed.

"Once the emergency is over, the light on Venice should not go out," Italian media reported.

"Venice needs attention. Now is the time to do it and if the MOSE serves as it should to avoid such disasters, then it must be finished."