The Environment Agency has issued four red flood warnings across Sussex in areas along the River Ouse.
This includes Lindfield Bridge near Haywards Heath, Freshfield Bridge to Sharpsbridge near Uckfield, and Barcombe Mills near Lewes.
Mock Bridge, near Shermanbury on the River Adur, is also in a red warning zone.
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People living in these areas have been told to move to a safe place and find higher ground.
Residents should also put flood protection equipment up around properties and businesses.
The Environment Agency also issued amber flood alerts on Tuesday morning.
They are in place across several areas including the Upper Ouse, Lower Ouse, River Adur east branch, Upper Arun, West Dean, Singleton, Charlton, East Dean and Chilgrove.
The warnings come after heavy rain lashed down on the county throughout Monday afternoon and into the evening.