The US government has said the oil spill in the Gulf of Mexico is the biggest accidental oil leak ever, as BP prepares its "static kill" operation to permanently seal its well. A new government estimate suggests BP's Macondo well leaked 4.9m barrels of oil before being capped last month. Scientists said only a fifth of the leaking oil - around 800,000 barrels - was captured during the clean-up. The well broke open after an explosion on a drilling well in April. These new figures may prove to be the definitive estimate of the scale of the disaster as BP prepares to seal its well, says the BBC's Steve Kingstone in Washington. The BP spill is greater than the 1979 Ixtoc I leak in the Gulf of Mexico, which gushed 3.3 million barrels. Only the intentional release of an estimated eight million barrels of oil into the Gulf by Iraqi troops during the Gulf War in 1991 was greater. On Tuesday, BP will continue tests to establish how quickly it can move to a procedure known as "static kill" - in which heavy drilling fluid known as "mud" will be used to force any remaining oil back into the reservoir. The well will then be sealed with cement. The well initially leaked about 62,000 barrels of oil per day, higher than any previous estimate of the flow. But as the well exhausted itself, the flow slowed to about 53,000 barrels per day. The flow ended on 15 July, when BP put a temporary cap on the well. Last week, BP reported a record $17bn (£11bn) loss, having set aside $32bn to cover the costs of the spill. 'Bottom kill' crucial The static kill, also known as "bullheading", takes place in three stages. First, a test determines if oil can be pushed back down the well into the reservoirIf that goes well, the static kill is begun by pumping in mud at low pressure. That may take all of Tuesday and possibly run into WednesdayThen, engineers will have to decide whether to pump in cement at the top of the well or wait and pump in cement from the relief well into the bottom of the damaged well.The relief well will reach the damaged well some time between 11 and 15 August. The permanent "bottom kill" will take anywhere between a number of days and a few weeks. The final casing has been cemented in place, which is the prelude to the last bit of drilling. An earlier effort, in May, to pump mud into the well using much of the same equipment failed because the pressure of the spewing oil and gas was too great. BBC News - BP gears up to halt oil spill confirmed 'world's worst'