BP has completed the first step in its latest attempt to completely stop the massive amount of oil spewing from one of its wells in the Gulf of Mexico.

In this image taken from video provided by BP, the arm of a remotely operated vehicle works at the Deepwater Horizon oil spill site in the Gulf of Mexico on Saturday. (BP/Associated Press)

Underwater robots on Saturday removed the cap from the gushing well, beginning a period of at least two days when crude oil will flow unabated into the sea.

Step two involves installing a tighter-fitting dome that is supposed to collect most if not all of the oil, sending it to ships on the surface, 1.6 kilometres above.

If the operation is successful, the oil could stop leaking as early as Monday, according to Thad Allen, the federal official leading the cleanup. However, BP senior vice-president Kent Wells said it could take up to a week to install the new sealing cap.

The current cap, installed June 4, was gathering only around half the spouting crude.

The U.S. Coast Guard said it's making the switch to the new cap now because of a forecast weeklong lull in the Gulf storm season.

With the old cap gone, as much as 19 million litres of oil is expected to flow from the damaged well over the next 48 hours.

Current U.S. government estimates of the spill range from between five to nine million litres a day, based on interpretation of a live video feed of the leak.

Containing the leak will not end the crisis that began when the Deepwater Horizon drilling platform exploded April 20, killing 11 workers.

Two relief wells are still being drilled in hopes that engineers can inject heavy mud and cement into the leaking well to stop the flow. That work is expected to be completed by mid-August. Then a monumental cleanup and restoration project lies ahead.

With files from The Associated Press