British oil giant BP started on Saturday to remove its current oil containment cap in the Gulf of Mexico to make way for a new, tighter-fitting one.

The London-based company said Saturday its engineers were using undersea robots to do the delicate work of removing the current cap to prepare for the installation of a new cap in the next four to seven days.

"Over the next four to seven days, depending on how things go, we should get that sealing cap on," BP's Senior Vice President Kent Wells said in a conference call from Houston on Saturday.

Wells said the new cap was expected to capture all the oil leaking from the underwater gusher, and it would, not like the current cap, prevent oil from billowing out from the bottom and top.

However, officials said the process of switching caps, which was approved by the U.S. spill commander Thad Allen Friday, would leave the oil gushing unrestrained in the Gulf of Mexico for four to seven days before the new cap was put in place.

BP also has another cap ready in case the new cap fails to work, Wells said.

Meanwhile, BP was hooking up a third oil-recovery vessel, Helix Producer, with hopes that it could begin siphoning oil by Sunday.

The new cap and oil vessel are part of BP's efforts to increase its oil-containing capacity with four vessels that can capture up to 80,000 barrels of oil a day and allow for more flexible connection and disconnection of the vessels in the hurricane season.

Meanwhile, work continued on the construction of BP's two relief wells, the best way of permanently stopping the oil leak. The first relief well has 60 feet to 70 feet to drill before it reaches the damaged Macondo well, Wells said.

BP and the U.S. government had expected the wells to be completed by mid-August, but a top BP official told the Wall Street Journal on Thursday that the well might be completed by the end of July.

The U.S. government scientists have estimated that 35,000 to 60,000 barrels of oil are gushing into the Gulf of Mexico everyday.

BP has in recent weeks been collecting between 24,000 and 25,000 barrels a day, largely by two vessels, the Discoverer Enterprise and the Q4000.

The spill, the worst in the U.S. history, began on April 20 after an explosion and fire aboard the BP-leased Deepwater Horizon drilling rig off the Louisianan coast, which killed 11 workers.

So far, the spill has affected all five U.S. Gulf states, costing BP more than 3 billion U.S. dollars to clean up the Gulf.