The agency missed an October 2005 deadline mandated by Congress to revise outdated water quality standards and says it will not be able to finish the job until 2011, the group said.

"A day at the beach is not worth a night at the hospital," Nancy Stoner, the director of group's clean water project, said during a telephone news conference five days before Memorial Day, the traditional beginning of the US beach season.

The Natural Resources Defense Council said it had served the EPA with a notice of its intent to sue in 60 days, as required by law, on Wednesday.

EPA officials were not immediately available for comment.

The lawsuit will seek to force the EPA to accelerate its timetable for setting new water quality standards and to strengthen those standards to "fully protect the public" from bacteria, viruses and parasites in beach water, the group said.

The EPA also needs to set standards for facilities that discharge contaminated water, such as sewage treatment plants, it said. In addition, the EPA should establish testing methods that allow public health officials to quickly decide whether to close beaches or advise people against swimming.

The current outdated standards may not protect beachgoers from serious illnesses such as hepatitis and encephalitis as well as a host of common stomach ailments and infections, the group said.

The EPA needs to put breakthrough technologies in microbiology -- the kind seen on TV crime scene shows -- to work detecting pollutants at beaches, said Dr. Joan Rose, director of Michigan State University's Center for Water Sciences.

"We are essentially using about 100-year-old methods, particularly when we monitor discharges that end up at our beaches," Rose said.

The elderly, children and people with weakened immune systems are particularly at risk from waterborne contaminants.

The Natural Resources Defense Council said experts estimate some 7 million Americans are made ill by contaminated water, including recreational and drinking water, each year.

Studies have estimated anywhere from 2 to 14 percent of people who go into the water at beaches become infected and serious outbreaks can send people to hospitals for treatment, Rose said.

The Natural Resources Defense Council advised beachgoers to find out whether their beaches are regularly monitored for water quality and to avoid those with visible discharge pipes or where the water smells or looks murky.

Urban beaches can be a particular problem after heavy rain because rainwater can wash pollutants into oceans, lakes and rivers.