The US Coast Guard has routinely approved requests by BP to use thousands of liters of toxic chemical a day to break up oil slicks in the Gulf of Mexico despite a federal directive that the chemicals be used only rarely on surface waters, congressional investigators said on Saturday after examining BP and government documents.

The documents show the Coast Guard approved 74 waivers over a 48-day period after the restrictions were imposed, resulting in hundreds of thousands of liters of the chemicals being spread on Gulf waters. Only in a small number of cases did the government scale back BP's request.

The extensive use of dispersants to break up oil gushing from BP's Deepwater Horizon raised concerns early on as to what long-term damage the toxic chemicals might be doing to the Gulf's aquatic life. That prompted the Environmental Protection Agency on May 26 to direct BP to stop using the chemicals on the water surface except in "rare cases."

But Rep. Edward Markey said on Saturday that the chemicals continued to be used extensively with Coast Guard approval, often at a rate of 23,000 to 38,000 liters (6,000 to 10,000 gallons) a day. A request was made and approved on June 13 to spread as much as 136,000 liters of dispersant, according to data obtained by Markey's Energy and Environment subcommittee.

The EPA directive "has become more of a meaningless paperwork exercise than an attempt ... to eliminate surface application of chemical dispersants," Markey wrote in a letter sent on Friday to retired Coast Guard Admiral Thad Allen, the government's point man on the spill.

Markey's office released the letter on Saturday as well as the documents his panel had analyzed. Markey said that instead of complying with the EPA directive, "BP often carpet bombed the ocean with these chemicals and the Coast Guard allowed them to do it."

The House investigators found that the Coast Guard routinely approved the chemical use, in some cases a week in advance. On five occasions the Coast Guard approved a BP request to use 22,700 liters a day over a weeklong period and "in many of these days BP still used more than double" the limit that was approved, Markey said in his letter.

A call to the BP press office in Houston was not immediately returned.

A spokesman on duty for the Unified Command Center in New Orleans did not have an immediate comment.

The chemicals break down masses of oil into small droplets that allow the oil to be more easily consumed by bacteria. But the chemicals also are toxic and it's not known what impact the large volume of chemicals being used against the BP spill might be having on marine life.

The EPA has acknowledged that there are tradeoffs and that some use of the chemicals are essential to combat the oil spill. The EPA directive issued in May concerned only surface dispersal of the chemicals. BP also has been using large amounts of chemicals near the ocean floor at the site of the damaged wellhead.

In recent weeks, little oil has been noticed on the Gulf surface, and scientists believe one reason for that might be the extensive use of the chemical dispersants.