A year after flooding swept through large portions of Britain causing billions in insured loss, a catastrophe modeling firm says more homes are at risk in Britain than a government report suggests.

Newark, Calif.-based Risk Management Solutions reported from London that as many as one in four homes in Britain are at risk of flooding caused by excessive rainfall and overflowing rivers.

RMS said that, using a 1,000-year flood scenario, an analysis of British properties employing high-resolution satellite imagery indicates that approximately ?600 billion ($1.18 trillion) of residential building value is exposed to flood risk.

However, RMS asserted in a statement that many people are unaware their home is at risk because the government's flood maps, provided by the Environment Agency, do not include the probability of drains being overwhelmed by extreme rainfall, which accounted for some two-thirds of the damage caused by last year's floods.

According to the Insurance Information Institute, UK flooding in June and July last year caused $4.48 billion in insured losses.

"The Environment Agency's maps only tell part of the story about flood risk in Britain," said Alice Stuart-Menteth, European flood model manager at RMS.

"When you take into account surface water flooding, the picture becomes more alarming because thousands more homes are exposed than the government suggests," she said.

Ms. Stuart-Menteth commented that last summer's events "clearly demonstrate that flooding from heavy rainfall in areas with inadequate drainage or from minor streams can have devastating consequences."

RMS said the current maps do not adequately assess how one catastrophic flood could simultaneously impact different locations, which has consequences for emergency response planners, owners of major utility companies and insurers.

The firm added that disaster plans developed on a local scale could be severely stretched if a flood affects a number of areas.

Reacting to Sir Michael Pitt's review of flood risk in the United Kingdom, RMS said it has submitted a response highlighting the need for a more comprehensive view of flood risk and recommending that the government and its agencies work with the insurance industry to devise mutually beneficial strategies for dealing with flood risk.

The Environment Agency said on its Web site that it is currently collecting information on the issue and providing information to local authorities, emergency services and utility companies. It is also asking the government for "strategic overview for all types of inland flooding." The agency noted that it is currently not responsible for issuing warnings about surface water flooding, only flooding of rivers and the sea.

It added that the 2007 floods were primarily caused by the combination of oversaturated land and overflow of the drainage system, not river flooding.