Tourists have been warned to stay away from Venice which has been hit by the biggest flood in more than 20 years.

A combination of heavy rain and high winds has led to the sea levels in the romantic Adriatic lagoon city rising 5ft above normal.

It was the fourth-highest level recorded over the past century and there were fears that the water could rise further if more bad weather materialises as forecast.

Many of Venice's streets, including the famous St Mark's Square, one of the city's lowest points, were submerged, forcing people to wade through knee-high water.

Ferry and water taxi services were suspended and some luxury stores were also flooded.

Boxes of tourist merchandise floated inside the swamped shops around the square and even the city's famed pigeons sought refuge on rooftops and windowsills.

Alarms went off at 6.37am to alert citizens, but many residents were taken by surprise because authorities had initially not forecast such a high water level.

Mayor Massimo Cacciari warned those planning a visit to "think again" adding that those already in the city should stay indoors.

"It's an exceptional acqua alta,' and unless you absolutely have to, don't go out," he said.

The city would not ask for a state of disaster to be declared since the flooding did not cause any deaths or trigger the collapse of buildings, the mayor added.

But the official overseeing Venice's architectural heritage warned the city was reaching its limits and said the water was receding at a worryingly slow rate.

The Foreign Office urged tourists to check the FCO website for any updates.

Workers were unable to install the raised wooden walkways used during flooding because the water rose too quickly under heavy rains.

In St Mark's Square tourists tried to stay dry by hopping on cafe tables and chairs sticking out of the water. The water was so high that one rowed a small speedboat across the wide square.

"There are very few streets that are water free," admitted city spokesman Enzo Bon.

There were no reports of damage to the city's architectural jewels, and the Culture Ministry was monitoring the situation, he added.

High water levels of between 40 to 51 inches above sea level are fairly commonplace in Venice and the city is flooded several times a year, although experts say that due to climate change it is on the rise.

In 1900, St Mark's Square flooded 10 times a year but now it floods as much as 60 times a year and the city is said to be sinking at the rate of 10cm every year.

The last time Venice saw such high waters was in 1986, while the all-time record was 76 inches in 1966.

That flood forced 3000 people to evacuate and damaged many historic buildings, but largely spared the city's art - which had long ago been removed to upper floors because of flooding by tides.

"In Venice, we know how to live with high water," said Mr Bon. "Of course there are some problems, because today's was an exceptional event."

To try and halt the problem the Italian government is spending more than £3.7bn on a flood defence system called Moses after the Biblical figure who parted the Red Sea.

The mobile barriers would be raised at times when high water is predicted but their installation has been slowed by controversy over claims that they affect the environmental stability of the area.

As a result the system is not due to be finished until 2011 but experts say the city is also at risk from high water because of the undermining of the islands by drilling for gas in the sea off the city.

The company building the barriers said, had the system been in place, the city would not have been flooded.

Giancarlo Galan, the conservative governor of the surrounding Veneto region, criticised Venice's centre-left administration for failing to prepare for the flood and for allegedly stonewalling the barrier scheme.

But Mr Cacciari insisted the city's experts had done a good job and had revised their forecasts well before the water came in.