An underwater robot has taken off the existing dome to make room for the new sealing cap.

But the operation may last between four and seven days - meaning oil will flow unimpeded into the sea.

Another system is also being linked up and may start collecting crude on Sunday, BP says. It is drilling two wells to intercept and block the leak.

It estimates the first of these wells will be finished in the first half of August, enabling the company to intercept the damaged well "and kill operations performed".

The explosion of the Deepwater Horizon rig in April killed 11 people. President Barack Obama has called the leak the biggest environmental disaster in US history.