Underwater robots have successfully removed a cap on the leaking oil well in the Gulf of Mexico, the first stage in fitting a tighter cap.
However, millions of litres of oil will flow freely until the new cap is fitted, an operation that could take four to seven days.
BP also says there is no guarantee of success in fitting a new cap at such a depth.
A fleet of 400 skimmers is in place to try to minimise the effects of increased oil loss.
The company has two other operations underway to trry to stem the leak.
A surface vessel, the Helix Producer, is being deployed to collect oil and two wells are being drilled to intercept the leak, which it is hoped will be a permanent solution.
On Thursday BP said the operation was ahead of schedule.
The leak began after an explosion and fire aboard the Deepwater Horizon drilling rig on 20 April. The rig sank two days later.