The Environment Agency has launched a £30m online map to tell homeowners in England and Wales for the first time what flooding risks they face.
Areas are given low, moderate or significant risk ratings according to location, predicted water levels, and type and condition of flood defences.
The agency says it will help councils make more informed planning decisions.
But critics say it could mean higher insurance premiums or even leave people unable to sell their homes.
More than two million properties are given risk ratings - though "low risk" could mean a less than a one-in-1,000 chance of being flooded in a year.
ENVIRONMENT AGENCY FLOOD MAPS
Visitors to the site enter their postcode or town name
A map appears complete with colour-coded representations
They show flood defences, risk of floods and extreme floods
They click on a more specific point on the map
They are told whether their risk is low, moderate or significant
The National Flood Forum said that although the maps would help potential buyers and property builders, it may lead to steeper insurance premiums and make it hard for people to sell their homes.
Community groups director Mary Dhonau said: "Homeowners that are highlighted at high risk are going to be pinned down to their homes.
"They're not going to be able to sell them very easily.
"The government needs to introduce a grant for those people to make their
homes flood-resilient and flood-proof."
Some insurance industry insiders agree that premiums may rise for homeowners who are not at risk, but who share a postcode with those who are - because that is the way the map has been put together.
But Association of British Insurers director general Mary Francis said the map "is a valuable resource for everyone involved in flood management".
"Property owners in areas at risk from flooding can take action to protect their homes and belongings, planning authorities can ensure development does not proceed in inappropriate locations of the floodplain, and insurers can continue to help and advise their customers on the basis of the best possible information."
The map's launch marks the start of the agency's annual campaign to alert the public to the risk of flooding and help them prepare.
Chairman Sir John Harman said: "There are practical steps we all must take to reduce the impact of flooding, and that starts with being better informed and using that information.
"We want to provide as much information as possible as soon as it is available to help people understand if they are at risk and what to do to prepare for flooding."
Sir John added: "Flooding is a natural phenomenon that will become more frequent as climate change leads to more extreme weather conditions.
"Our flood map is the most detailed and comprehensive available for the whole of England and Wales.
"We are encouraging everyone, particularly those living and working in areas prone to flooding, to use it.
"Find out whether or not you and your property, your family and friends, are at risk from flooding and decide what action you need to take - don't leave it too late."
Yet again the Environment Agency has declared the whole of the area around the Wash as a flood risk. Anyone with even the slightest grasp on the geography of the area will know that we have the most prolific system of drainage anywhere in the UK - a network of dykes, drains and sluices. All this map achieves in the short and medium term is turning away anyone who is thinking of moving to the area.
Steve Tilley, Boston, UK
We bought our new build home a year ago. The environmental survey done for the mortgage indicated the house to be less than 250m from the flood plain of the River Dane. However, due to the steep sided valley which holds the river we are not at risk. The house is about 50 feet above the flood plain which is tightly constrained by the steep slope. I explained all this to my insurers and they were satisfied and my premium was not affected. The flood plain data came from the Environment Agency, and I am pleased to see that their revised online maps confirm that my house is still outside of the extreme flood risk area from the River Dane.
John F, Congleton, UK