Oil from the Gulf of Mexico spill has forced authorities to close or post water quality warnings at one in five beaches in the 100 days since the crisis began, a report said Wednesday.

Forty-nine of 253 stretches of beach in Louisiana, Alabama, Mississippi and Florida have had to close or post warnings for swimmers as a result of the spill, the report by the Natural Resources Defense Council (NRDC) said.

The combined number of days beaches were closed or posted water quality warnings came to nearly 2,000, nearly ten times the number last year, it said.

Among those closed by the spill was Gulf Shores Public Beach in Alabama, which in past years has been awarded the highest possible rating of five stars for cleanliness by the NRDC.

Millions of barrels of oil have spewed into the Gulf since the April 20 explosion on the Deepwater Horizon rig that killed 11 workers and sank the structure 50 miles (80 kilometers) off the Louisiana coast.

Although it has few beaches, Louisiana fared the worst of the Gulf states.

Eleven of the 28 Louisiana beaches that are monitored regularly were closed for a combined total of 793 days because of the spill, the report said.

In western Florida, 16 of 180 beaches that are regularly tested for water quality have posted advisories because of oil from the spill. Last year, no advisories were posted at Florida's Gulf beaches for any reason.

And in Mississippi, 16 of 20 monitored beaches have issued 430 days of oil-related advisories this year, compared to 57 advisory days for any reason last year.

Beaches in Texas, on the other hand, have so far escaped unscathed by the oil, with no beaches having been closed or placed under advisory in the westernmost of the Gulf states.

Beach tourism and recreation contributed 26.5 billion dollars to the economies of the five Gulf states in 2004, the report said.

But "the specter of oil 'mousse', tar balls, tar mats and even liquid oil on the sand and in the water" is keeping tourists away this year, the NRDC said.