Repairs costing between £20m and £50m may be needed to prevent houses in and around Derby being flooded, the Environment Agency has revealed.
Staff are currently checking the quality of flood defences in houses from Duffield to the River Trent.
The Agency estimates 6,500 homes may be at risk.
Bosses have warned that some areas of Derby would be flooded if it suffered a similar amount of rain to that experienced in Gloucester in 2007.
Best options
There has been no serious flooding in Derby since 1965.
The Environment Agency said flood defences already in place were capable of withstanding most years' river levels but a current study was looking at managing the risks from flooding for the next 50 years and beyond.
Both banks of the River Derwent were at risk of flooding as well as areas including Duffield, Little Eaton, Elvaston, Draycott and Shardlow.
The Agency spends about £1m a year maintaining existing flood defences and rivers in Derbyshire.
A spokesman said: "We recognise that the estimate of £20m to £50m is very broad.
"This is because until the Lower Derwent Strategy study tells us what needs to be done and where, and what are the best options for resolving any issues, we just don't know what the cost will be.
"The report should be ready for consultation this summer and by then we should have a better idea."
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