Beleaguered energy giant BP was hit with further bad news this morning as it emerged dangerous arsenic levels have been found in seawater around the Gulf of Mexico.

British scientists warned that the oil spill is increasing the level of arsenic in the ocean, and could further add to the devastating impact on the already sensitive environment.

BP’s Deepwater Horizon rig has been spilling between 3,681,500 litres and 911,454,000 litres of oil into the sea per day since it exploded on April 22.

The spill is already being labeled as America’s worst environmental disaster and has turned into a economic and PR nightmare for the British company.

Seventy-five days into the spill, the oil has fouled some 715km of shoreline in four southeastern US states, killed wildlife and put a massive dent in the region’s multi-billion-dollar fishing industry.

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The clean-up operation, which has already cost, $US3.12 billion ($3.7 billion), is expected to rise even further after efforts were hampered by technical setbacks to cap the leak and adverse weather conditions.

In a further blow, an operation to permanently cap the ruptured well on the seafloor far below the surface cannot begin until engineers finish drilling relief wells, in mid-August at the earliest.

Imperial College London researchers warned the effect on the environment could worsen unless clean up efforts were hastened.

Researchers published a study which found oil stops the ocean’s natural filtering process of arsenic.

They said the arsenic then gets “magnified” up the food chain, as fish eat small amounts of the deadly poison and may eventually impact humans, researchers said.

Professor Mark Sephton said arsenic, which is found in seawater, was normally filtered out of the ocean when it combined with sediment on the sea floor.

“But oil spills stop the normal process because the oil combines with sediment and it leads to an accumulation of arsenic in the water over time,” he said.