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BP have got permission from the US government to test the new temporary well cap they've fitted to the Macondo oil well and see if it can handle the pressure of oil pumping out.

Oil has been gushing into the Gulf of Mexico ever since the Deepwater Horizon oil rig over the Macondo oil well exploded on April 20.

Up to 170 million gallons (651 million litres) of oil have flooded into the sea so far causing terrible damage to the environment.

Wildlife and people are going to be affected by the leak for many years to come.

The new cap - which is a 75-ton metal stack of pipes and valves - was lowered onto the well on Monday. The idea was to either bottle up the oil inside the well machinery, or capture it and funnel it to the surface.

But before BP could test the new cap, the US government stepped in because they were worried that the buildup of pressure from the gushing oil could actually break the walls of the well and make the leak worse.

But now the US government have given BP the go ahead to start testing the new cap.

If the cap works, it will mean BP can stop the oil from gushing into the sea, either by holding all the oil inside the well machinery like a stopper or, if the pressure is too great, channelling some through special lines to ships collecting the oil.

The test began with BP shutting off pipes that were funnelling some of the oil to ships on the surface so the full force of the oil gush went up into the cap.

Then deep-sea robots began slowly closing, one at a time, three openings in the cap that let oil pass through. The aim is to eventually completely stop the flow of oil.

Engineers will be taking pressure readings the whole time to find out whether the well is intact.

The first two valves shut off like a light switch, while the third works more like a dimmer switch and takes longer to close off.

Last night, BP said it had found a leak on the line attached to the dimmer switch type valve, and was repairing it before moving forward.

Even if oil cap works there's a huge amount of work to do before the oil is cleaned up.