Downing Street has denied the prime minister misled Parliament when he said flood defence costs had been protected in the Spending Review.
On Wednesday, in the wake of the flooding in Cornwall, David Cameron said spending had been protected.
When told it had fallen from £2.3bn to £2.1bn the prime minister's spokesman said it was "broadly the same" although overall costs had been cut by a third.
Diana Johnson MP said Defra faced 28% cuts and called for a Commons debate.
The Labour MP for Kingston-upon-Hull North said: "In the light of the dreadful floods in Cornwall the prime minister said yesterday that spending on flood protection was going to be protected in the Comprehensive Spending Review.
"I understand there is going to be cuts of up to 28% in the flood protection budget."
She said a debate was needed "so the prime minister's statement can be corrected".
But Commons leader Sir George Young told her: "The statistics which my right honourable friend gave yesterday were correct.
"If you were listening to the Today programme you would have heard the chairman of the Environment Agency confirm that those were indeed the figures for the four-year period concerned."
Sir George said there would be a written ministerial statement on the floods in Cornwall on Friday.
Mary Dhonau, chief executive of the National Flood Forum, said the government must commit more money to help flood victims.
"No matter what spin David Cameron puts on it, it's a woefully insignificant amount because so many people are now at risk of flooding," she said.
"Think back to the 2007 floods - that was all over the country really and as far as I'm concerned there's nobody really that's not at risk of flooding."
The Environment Secretary Caroline Spelman is visiting Cornwall to see the aftermath of the floods.
A massive clean-up operation is under way after severe flooding resulted in more than 100 homes being evacuated.
Torrential rain overnight from Tuesday into Wednesday left some properties under 3ft (1m) of mud.
More rain overnight led to flood warnings and watches remaining in place on a number of rivers.
Liberal Democrat MP for North Cornwall, Dan Rogerson, said lessons must be learned from what happened.
Lord Smith, chairman of the Environment Agency, said the agency would receive £2.1bn in government funds over the next four years, enabling it to complete existing projects.
He told the BBC: "Flood is always a traumatic thing for everyone whose home or business is affected in this way.
"The good news is that now the flood waters are receding in Cornwall and now the clean-up operation has to get under way.
"Where we will have difficulty, I fear, over the coming couple of years is in starting new flood defences that might have been possible."
Meanwhile an emergency flooding exercise is taking place in Camberley, Surrey, on Thursday.
The Pitt review into the 2007 floods recommended a national flood exercise be held to test new practices. This is the pilot to that exercise.
St Blazey, St Austell, Mevagissey and Lostwithiel were the worst-hit areas of Cornwall as people were trapped in their cars and homes by the rising floodwaters, which reached up to 6ft (2m) deep in places.
There were no reports of serious injuries but residents were evacuated and schools around the county were closed.
The county's transport network was hugely disrupted and about 25 vehicles had to be recovered from floodwater.
Train services in and out of the county were also affected by landslides, with the mainline track between Penzance and London Paddington closed for several hours on Wednesday.
The Newquay to Par branch line remains shut after a storage tanker was washed down on to the railway track and a landslip caused subsidence under the lines.
National Rail Enquiries said buses would replace trains until early next week.
A 700-year-old bridge over the river in Lostwithiel is currently shut as engineers examine it for damage.
Cornwall Council set up an emergency shelter at Polkyth Leisure Centre in St Austell, where 200 blankets and 200 groundsheets were donated by local charity ShelterBox.
The international aid charity specialises in delivering aid to crisis-hit places overseas but sent blankets to the emergency shelter in Polkyth, the first time it had assisted people in its home county.
The Eden Project, which was hit by 3ft (1m) of floodwater in places, said it expected to be closed for at least a week.
Cornwall Council Leader Alec Robertson said he had spoken to the prime minister on the telephone and said he had expressed his sympathy and offered government help.