"We are talking about hundreds of people being affected by the flooding at this present time - something like between 500 and 600 homes around Todmorden," said Senior Divisional Officer Simon Pilling, of the West Yorkshire fire service.
As clean-up operations continue, homeowners and businesses in the worst affected areas, of West Yorkshire and County Durham, have been left sifting through ruined possessions and property.
About 80 families in West Yorkshire were evacuated from their homes, hundreds took refuge in town halls and drivers had to be rescued from cars stranded by the flood waters which rose without warning on Sunday morning.
Meanwhile, on Monday, the Environment Agency said only a handful of flood warnings remained across several counties, as record river levels returned to normal.
Insurers in Todmorden, where more than 500 homes were engulfed, estimated the clean up bill could reach £12m.
'Extensive damage'
Richard Guthrie, managing director of Donald Riggs Insurance Brokers, said his staff had been handling a mountain of claims.
He said: "We have a lot of clients in the town and I can quite easily see the overall cost of the flooding reaching £12m without any trouble.
"Residents are not only facing extensive damage to property, but they are facing costs for alternative accommodation and the clean-up.
"Many businesses have been greatly affected and we are going to see claims for loss of profits on top of claims for damage to stock and premises."
Widespread damage
About 8,000 homes were left without electricity in Lincolnshire, hazardous driving warnings were issued and trains between London and the north faced delays after disruption caused by floodwater covering the tracks.
"We are talking about hundreds of people being affected by the flooding at this present time - something like between 500 and 600 homes around Todmorden," said Senior Divisional Officer Simon Pilling, of the West Yorkshire fire service.
He said: "Mytholmroyd was the first town to be hit, where we had to evacuate around 50 homes.
"We are just about getting to the Hebden Bridge area to assess the extent of the flooding."
Mud, oil and rubbish were washed into the streets.
Some families have returned to find their homes wrecked by the floods, the force of the water moving furniture and fridge freezers.
More rain expected
Mr Pilling said the rain had eased off on Sunday, but more was forecast for the next two days.
"Luckily we have not had any reports of casualties, although police divers carried out a search for a householder thought to be trapped in his cellar," he said.
"The topography of the area means that a lot of water has been running off the steep valley sides, which has been adding to the problems caused by the rising river levels."
Firefighters had to rescue more than 200 sheep which were in danger of drowning near Seaton.
Meanwhile, floodwater closed the main east coast railway line at Croft near Darlington, in County Durham, blocking Intercity trains for most of Sunday.