Experts warned large swathes of the Sussex coast could be hit today by gale-force winds and extremely high tides.
The Environment Agency issued an urgent flood warning for the coastline between Chichester Harbour and Selsey Bill and said the rest of the Sussex coast should also be on high alert.
Extremely high tides were expected at 1am and gale-force winds were also predicted last night.
The Environment Agency warned tides were likely to breach sea defences and did not rule out knee-deep floods in towns and villages in the Chichester and Selsey areas.
It warned people living in low-lying areas near the coast to be vigilant as their homes could be hit.
Motorists were also warned to check conditions before setting out and to avoid coastal roads if possible.
Most inland rivers should not be affected but many tidal rivers will also see high water levels and a flood watch warning has also been issued for the Lower River Ouse.
Andrew Gilham, Sussex flood risk manager for the Environment Agency, said: "We are urging people to be aware and prepared for flooding should conditions worsen.
"We will continue to closely monitor the situation throughout today and tomorrow.
"We want householders to be prepared, register for our flood warning service and find out the measures they can take to make their property flood resistant."
A spokesman for the agency said: "It's impossible to say how high the floods will be so we just want to make sure people are alert.
"There have been times in the past when tides have breached defences and if people have property near defences they should keep a careful eye on what's happening."
The potential floods are a result of the peak of the spring tides coupled with strong south-westerly winds. The Environment Agency provides a 24-hour flood helpline on 0845 988 1188 which gives localised flood warning information advice.
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