Some one billion people, a sixth of the world's population, are currently under threat from major flooding and the number could double unless measures are taken, said Janos Bogardi, UNESCO water services division chief.
"The small island states are most in danger, their very existence is in danger, even if the number of people living there is not very high," Bogardi told AFP.
"Other events linked with floods could lead to the disappearance of some states like the Netherlands. Rising sea levels is leading to rising river levels," he said. "The figure of two billion is a conservative estimation."
His warning was issued ahead of the opening on Tuesday of the new UN University Environment and Human Security Institute in Bonn, Germany.
The causes, Bogardi says, are extreme weather that develops as the climate changes, rising sea levels, continuing deforestation and a rise of the number of people working in flood-prone areas.
Most at risk is Asia, where more than 400 million people have been exposed to floods each year on average over the last two decades and where flood damage totalled 136 billion dollars between 1987-1997.
"Other events linked with floods could lead to the disappearance of some states like the Netherlands"
UNESCO water services division chief
From 1971 to 1995, floods affected more than 1.5 billion people, or 100 million a year, a total that includes 318,000 killed and more than 81 million left homeless.
The number of major flood disasters has risen relentlessly. There were six in the 1950s; seven in the 1960s; eight in the 1970s; 18 in the 1980s; and 26 in the 1990s.
Bogardi, who is also director of the institute, said it was vital to increase the global capacity to monitor and forecast extreme weather events.