Under immense pressure to plug its catastrophic American oil leak, BP is preparing for a hazardous last-ditch salvage operation that risks making the gush of crude into the Gulf of Mexico even heavier if its robotic submarines fail in an inch-perfect exercise to cut through a broken pipe a mile beneath the ocean's surface.

After the failure of a "top kill" effort to stuff the leak with mud and rubbish, BP's managing director, Bob Dudley, stressed the difficulty of a new plan to pipe spurting oil to a ship on the ocean's surface, describing it as highly challenging for engineers who will be asked to perform "the equivalent of open heart surgery on television for everyone".

The under-fire company faced calls from Louisiana lawmakers to make an immediate $1bn investment to protect threatened wetlands and estuaries on the Louisiana coast. The White House's energy tsar, Carol Browner, warned that the giant, rapidly expanding oil slick caused by an explosion on BP's Deepwater Horizon oil rig was‚ "probably the biggest environmental disaster we have ever faced".

Since becoming severed from BP's destroyed offshore rig on 30 April, a broken drilling pipe known as a "riser", which carries oil from an underground seam through the ocean floor, has been kinked downwards in the water. BP is deploying diamond-rimmed saws to cut through the pipe below this kink, in the hope that it can create a sufficiently clean break to attach a cap capable of siphoning oil to the ocean's surface, known as a‚ "lower marine riser package".

Experts worry that by removing the bend in the pipe, any remaining resistance to the flow of oil into the ocean will be eliminated. Philip Johnston, an engineering professor at the University of Alabama, said: "This operation is scary for BP.

"When they cut the riser the rate of flow from the well, which is already truly horrific, will get significantly bigger. If they can't then get the cap on, it will make things worse."

BP's chief operating officer Doug Suttles told a press conference on Saturday that he could not guarantee the attempt would work, and even if it did it would only capture "the majority" of the oil. Following the first Gulf war in 2001, a similar capping procedure was used to extinguish the well fires lit by the retreating Iraqi army. However, it has never been attempted 1,500 metres (5,000ft) below sea level.

President Obama admitted that cutting the riser pipe "is not without risk", saying: "That is why it was not activated until other methods had been exhausted."

On the Louisiana shore, dozens of shrimpers, fishermen, holiday-home owners, restaurateurs and hoteliers have filed lawsuits against BP for the damage to their earnings caused by the slick.

Local people expressed fury at the British company's apparent inability to stem the flow. Claude Marquette, a retired doctor in his boat at the fishing port of Venice Marina, said: "It's been a screw-up from day one. Nothing was at the ready and no one was in a position to respond."

Father Gerry Stapleton, a priest at St Patrick's Catholic church in nearby Port Sulphur, said: "Even the government seems powerless and all the experts. If these people can't stop it, then who in the name of God can?"

There is particular acrimony that BP's initial estimate of the leak at 5,000 barrels of oil a day dramatically understated the scale of the flow, which is now put at 12,000 to 19,000 barrels a day. Ed Markey, the Democratic chairman of a congressional energy committee investigating the disaster, said BP was "either lying or they were incompetent".

Speaking on CBS's Face the Nation talkshow, Markey described the leak as criminal‚ and added: "I have no confidence whatsoever in BP."

The waters affected by the spill are among the most sealife-rich around the US. In 2008, commercial fishermen landed 600m kg of finfish and shellfish in the Gulf of Mexico, generating $659m. Oyster fishermen worry that not only the oil but the chemical dispersants used to break up the slick will poison their harvest for months and years to come.

Mary Landrieu, a Democratic senator for Louisiana, said BP should immediately commit $1bn to safeguarding the coastline: "While we may not be able to plug the leaking well right away, there is nothing that should stop us from getting help to the Gulf coast immediately."

Obama ordered government and contractor clean-up resources in areas affected by oil to be tripled.

The Washington Post termed the leak "the well that will not die". Many critics of the response to the crisis have urged the president to put the armed forces in charge, taking over operations from BP.

But Mike Mullen, chairman of the joint chiefs of staff, said that the military did not have the necessary expertise.

"It's really not for us to lead right now because of the technical challenges," said Mullen. "The technical lead for this in our country really is the [oil] industry."

Even if BP succeeds in its latest effort to siphon oil, the operation faces a meteorological hazard in the hurricane season, which starts at the beginning of June. David Summers, a professor of mine engineering at Missouri University of Science and Technology, said it would be impossible to leave a containment vessel collecting oil on the open sea if severe weather sweeps into the Gulf.

As it attempts a seal, BP will need to pump large volumes of warm water towards the leak to avoid the formation of ice crystals, which stymied a previous attempted fix. Summers said: "This may be the best option they've got but because they're going to be dragging through the flow of oil as they cut, it's going to be difficult to achieve a smooth, straight cut."

If BP cannot halt the flow, oil will continue to gush until two relief wells are ready to intercept the flow of oil by intersecting with the original bore beneath the ocean floor.

One of these has reached a depth of 3,600 metres (12,000ft), while the other is down to 2,500 metres, but they need to go as far as 5,500 metres before they can tap into the oil, which may not happen until August. By then, the slick on the ocean's surface, estimated at 18m to 40m gallons of oil, could have doubled in size.