Flood warnings are in place across the country as heavy rain lashes down, with more expected to be on the way.
The main dual carriageway through Cornwall has been closed near Hayle where shops and homes have already been affected.
There are 18 flood warnings in place across England, mainly across the South, South West and River Severn.
A yellow warning for rain in the most affected areas remains in place until 12:00 on Friday.
Flooding has also caused extensive disruption to trains through the New Forest, affecting services from London Waterloo to Southampton, Bournemouth, Dorchester and Weymouth.
Floods in Hedge End, near Southampton, have also caused railway lines to be blocked.
A landslip at Liss, in Hampshire, blocked the line to Portsmouth and there is also disruption to services between Fareham and Eastleigh.
There is also significant traffic disruption in Dorset because of flooded roads, including on the A35 bypass near Dorchester.
Devon and Cornwall Police have declared a major incident in Hayle, with the A30 closed in both directions west of Camborne.
Water has made its way into about 50 properties in Hayle so far, and it has affected a number of local businesses, including Angove Sports.
"We have got quite a lot of the stock off the floor, but [the water] did come in quite quickly," said owner Andrew Angove.
"It was the cars and lorries going past so quickly that made a wave that came right through, pushing it through the shop door," he added.
Damian Knight, owner of the Cornubia Inn in the centre of Hayle, said: "We've been really, really lucky.
"The water came right up to the top of my front step, but it didn't come into the pub at all."
"Unfortunately there are others around us who aren't so lucky," he added.
This was the #A30 near #Hayle after torrential rain swept across the area early— BBC Radio Cornwall (@BBCCornwall) December 19, 2019
❌ A30 closed at Loggans Moor
❌ “Numerous” Hayle businesses affected say @CornwallFRS
❌ @DC_Police urge drivers to avoid area
Video by Elliot Lloyd - full reports @laurencereed @BBCSpotlight pic.twitter.com/RiycC32L12
There are also flood warnings near Tewkesbury, Worcester, Southampton, Crawley, Weymouth, Yeovil and Exeter.
People are being urged to stay away from floodwater and not to attempt to cross deep water.
Three out of four people said they would attempt to drive through floodwater "despite it being the leading cause of death during a flood", the Environment Agency said.