US government and BP officials are warning that the blown-out oil well causing an environmental disaster on the Gulf Coast may not be stopped until August as the company begins preparations on a new attempt to capture the leaking crude.

The disaster, in its 42nd day on Monday, is already the largest oil spill in US history and officials are calling it the country's biggest environmental catastrophe.

In the wake of a devastating failure this weekend to plug the BP well with the tricky "top kill" operation, BP chief executive Tony Hayward said the Gulf of Mexico leak may not be stopped for two months.

"There's no doubt that the ultimate solution lies on the relief well, which is in August," he said on Sunday as he toured the Louisiana fishing hub of Venice, largely idled by the spill but reliable way to intercept and cap the leaking well that ruptured with a deadly rig explosion on April 20.

In the meantime, Hayward said BP needs "to be in the mind-set of containment in the sub-sea, containment on the surface and defending the shoreline, in a very aggressive way."

The Gulf spill has surpassed the Exxon Valdez disaster off Alaska in 1989 as the worst US oil spill, with an estimated 12,000 to 19,000 barrels (504,000 to 798,000 gallons/1.9 million to 3 million liters) leaking per day.

Right now, BP is preparing a containment cap to place on top of a lower marine riser package (LMRP), which is a piece of equipment that sits atop the failed blowout preventer at the seabed one mile (1.6 km) below the surface.

The White House said the company would begin cutting a pipe that rises out of the so-called LMRP on Monday or Tuesday.

If the containment operation works -- and BP expects to know later this week -- then at least some of the leaking oil could be piped to the surface.

But even President Barack Obama's administration, facing a barrage of criticism that they have been slow to respond to the crisis, is looking toward August and the relief well solution.

Elections loom

"There could be oil coming up until August," top White House energy adviser Carol Browner told CBS's Face The Nation on Sunday. "We are prepared for the worst."

The oil spill has unleashed a surge of public anger that poses a major domestic challenge to Obama and his party as they face congressional elections in November.

The Gulf Coast is one of America's richest ecosystems and a vital breeding ground for a $6.5 billion seafood industry.

Kuzma Tesvich, an oyster fisherman attending the Seafood Festival in Belle Chasse, Louisiana, on Sunday, said his livelihood could be hurt for several years.

"If this goes on, if our oysters are totally killed, it would be at least three years before we can come back, it could be a lot longer," Tesvich said.

The spill has already shut down a good chunk of the fishing business. For the festival, Tesvich brought in oysters from unrestricted waters east of the Mississippi River.

Louisianians, whose anger has only grown after the top kill failure, want the government to do more. They remember the fumbled response of former President George W. Bush's government to Hurricane Katrina in 2005.

"They should be down there seeing what it's doing, everything that it's killing. They should be down here supporting the people of Louisiana," said Cathy Van Der Griff as she visited New Orleans' famous Cafe du Monde.

Obama, who made his second visit of the disaster to the Gulf last Friday, is sending three of his top energy and environmental officials back to the Gulf this week. – Reuters