The US Government has declared the BP oil spill in the Gulf of Mexico the world’s worst oil leak caused by an accident.

Government estimates suggest BP’s Macondo well leaked 4.9 million barrels of oil, which is greater than the 1979 Ixtoc I leak that gushed 3.3 million barrels in the Gulf of Mexico.

The world’s worst spill was in the 1991 Persian Gulf War when retreating Iraqi forces opened oil pumps, causing the release of six million barrels, according to the US Environmental Protection Agency.

Meanwhile, BP will carry out a crucial test before injecting mud and cement into the Gulf of Mexico oil well in one of its final steps to permanently seal the ruptured well.

The well broke open after the Deepwater Horizon rig exploded, killing 11 workers in April.

Japan’s Mitsui, which has a 10% stake in the well, said it has received a bill for $480m from BP to pay clean-up costs.