NEW ORLEANS - The BP oil leak could be completely contained as early as Monday if a new, tighter cap can be fitted over the blown-out well, the government official in charge of the crisis said Friday in some of the most encouraging news to come out of the Gulf in the 2-1/2 months since the disaster struck.

If the project planned to begin this weekend is successful, it would simply mean no more oil would escape to foul the Gulf of Mexico. The well would still be busted and leaking - workers would just funnel what comes out of it to tankers at the surface. The hope for a permanent solution remains with two relief wells intended to plug it completely far beneath the seafloor.

"I use the word 'contained,'" said retired Coast Guard Adm. Thad Allen. "'Stop' is when we put the plug in down below."

Crews using remote-controlled submarines plan to swap out the cap over the weekend, taking advantage of a window of good weather following weeks of delays caused by choppy seas.

The cap now in use was installed June 4 to capture oil gushing from the bottom of sea, but because it had to be fitted over a jagged cut in the well pipe, it allows some crude to escape into the Gulf. The new cap - dubbed "Top Hat Number 10" - is designed to fit more snugly and help BP catch all the oil.

During the installation, the gusher will get worse before it gets better. Once the old cap is removed, oil will pour into the Gulf unhindered for about 48 hours while the new one is put in place, Allen said.

BP also worked on Friday to hook up another containment ship called the Helix Producer to a different part of the leaking well. The ship, which will be capable of sucking up more than 1 million gallons a day when it is fully operating, should be working by Sunday, Allen said.

The government estimates 1.5 million to 2.5 million gallons of oil a day are spewing from the well, and the existing cap is collecting about 1 million gallons of that. With the new cap and the new containment vessel, the system will be capable of capturing 2.5 million to 3.4 million gallons - essentially all the leaking oil, officials said.

The plan had originally been to hook up the Helix Producer and install the new cap separately, but the favorable weather convinced officials the time was right for both operations.

"Everybody agrees we got the weather to do what we need," Allen said. He said the calm weather is expected to last seven to 10 days.

In a response late Friday to Allen's request for detailed plans about the new cap, the Helix Producer and the relief wells, BP managing director Bob Dudley confirmed that the leak could be contained by Monday.

But Dudley included plans for another scenario, which includes possible problems and missteps for the installation of the cap that would push the work back until Thursday.

The past 80 days have seen the failure of one technique after another to stop the leak, from a huge containment box to a "top kill" and a "junk shot." The latest approach is not a sure thing either, warned Louisiana State University environmental sciences professor Ed Overton.

"Everything done at that site is very much harder than anyone expects," he said. Overton said putting on the new cap carries risks: "Is replacing the cap going to do more damage than leaving it in place, or are you going to cause problems that you can't take care of?"