A top Obama administration official warned Sunday that the Gulf of Mexico oil spill might not be stopped until late summer after BP’s latest attempt to plug the leak failed.

The “American people need to know” that it’s “possible we will have oil leaking from this well until August, when the relief wells will be finished,” said Carol Browner, the White House energy advisor.

Browner said on CBS that Energy Secretary Steven Chu and a team of scientists on Saturday essentially put a halt to BP’s attempt to cap the spewing well through a process known as “top kill.” The administration team worried that the increasing pressure from heavy drilling mud being forced into the well to seal it actually would make the leak worse.

The worst oil leak in U.S. history is now in its 41st day. It is sending 504,000 to 798,000 gallons a day into the Gulf of Mexico, according to estimates by a government panel.

Asked whether U.S. officials told BP to stop the top-kill process, Browner said, “We told them of our very, very grave concerns” that it was dangerous to continue building up pressure in the well. She said that company officials listened.

With the failure of the top-kill procedure, BP Managing Director Robert Dudley said Sunday the company was moving to a new procedure that had a better chance of working, although he admitted that even if successful, it would not completely cut off the torrent of oil spilling into the gulf.

BP officials had said top kill — which involved pumping heavy drilling mud to seal the leak — had a 60% to 70% chance of working.

“There is no certainty, but we feel like the percentages are better that we’ll be able to contain the oil,” Dudley told Fox News Sunday about the lower-marine-riser package that will attempt to cap the well’s failed blowout preventer. “The question is how much of the oil will we be able to contain, and the objective is to try to collect the majority through this vessel.”

Dudley said BP officials were “disappointed” that the top-kill effort failed and “immediately” started on the new effort. The process was less technically complicated than top kill, he said. It involves using robots to cut the leaking pipe cleanly and then installing a cap to allow most of the oil to be pumped up to ships on the surface.

A similar effort this month to place a containment dome over the leak failed because gases coming from the well froze at those deep sea depths, preventing the dome from being placed properly. Dudley said that BP “learned a lot” from that attempt and this time would pump warm sea water down the pipe to keep the gases from freezing.

But Browner said cutting the pipe could increase the amount of leaking oil by 20% and that some oil would continue leaking after the cap was in place.

“If it’s a snug fit, then there could be very, very little oil. If they’re not able to get as snug a fit, then there could be more,” Browner said. “We’re going to hope for the best and prepare for the worst.”

Browner defended the Obama administration’s response to the spill. She said it had forced BP to take steps the company resisted, particularly drilling a second relief well in case the first one did not work. But those wells will not be finished until the end of August.

Dudley said on ABC that those wells will “certainly” be “the end point on this game.” But he said company officials were optimistic the latest procedure would work.

“We failed to wrestle the beast to the ground yesterday,” he said. “We’re going to go in and put a cap on it.”

Rep. Edward Markey (D-Mass.), chairman of one of the congressional committees investigating the spill, said the relief wells must work, which was why it was important for the administration to force BP to drill a second one as a backup. But he accused BP of “boosterism” for its continued optimism about stopping the leak through other methods.

“BP has been making it up as they go along the whole way. They have not been prepared. They do not know for sure what the result is going to be of anything which they are doing,” Markey said on “Face the Nation.” “I have no confidence whatsoever in BP.”

jim.puzzanghera@latimes.com