BP embarked upon a high-risk "top kill" procedure using drilling mud last night to cap the catastrophic gush of oil in the Gulf of Mexico, as it faced fresh accusations of shortcuts in the hours before an explosion destroyed the Deepwater Horizon rig.

After hesitation by top BP executives as they analysed data from robot submarines at the site of the leak, and under intense pressure from the Obama administration, the US coastguard gave the go-ahead for the operation to pump a cocktail of mud and heavy fluid at high pressure into the Macondo well, 50 miles off the Louisiana shore.

Underwater TV cameras showed a live feed of oil billowing out while BP's heavy machinery moved into position. Chief executive Tony Hayward warned that the operation, never before attempted at a depth of 5,000 feet (1,500 metres), had only a 60% to 70% chance of success and could take several days. Barack Obama described the disaster as "heartbreaking" and expressed hope it would work. "If it's successful, and there's no guarantee, it should greatly reduce or eliminate the flow of oil now streaming into the Gulf from the sea floor," said the president.

After a boat trip to see the damage, Louisiana's governor, Bobby Jindal, displayed vivid photos showing that oil was killing cane plants along the state's offshore marshes, an area which he described as "the nursery of the gulf". "We've been fighting this oil nearly a month now, requesting resources. Too often, the response has been too little, too late," he said; absorbent booms to capture oil were becoming saturated. "We can't afford to wait another 24, or 48 hours."

The slick is estimated to cover 16,000 sq miles of ocean, It began on 20 April when an explosion and fire destroyed BP's rig, killing 11 people. Since then efforts by BP to stem the flow, first by placing a dome on top of the leak, have come to nothing.

David Summers, a professor at Missouri University of Science and Technology, said it would be clear within days whether the "top kill" procedure had worked. The method was straightforward and would have been started much earlier, were it not for the inaccessibility, he said: "It's relatively simple and been done many times before, but not at this depth."

In a massive operation, 22,000 people and 1,100 vessels are tackling the slick. BP's top executives are monitoring events from a control centre in Houston. But as investigations begin into the cause, BP is facing accusations of "short cuts" in the hours before the rig blew up.

In official hearings in New Orleans, several workers who survived raised questions about a decision shortly before the explosion in which rig bosses displaced heavy mud with salt water in the pipe rising from the seabed, potentially hampering the rig's ability to withstand pressure from the ocean depths.

At yesterday's hearings Truitt Crawford, a roustabout on the rig, told coastguard investigators: "I overheard upper management talking, saying that BP was taking shortcuts … this is why it blew out." Another witness, Doug Brown, chief mechanic on the rig, said there was a "skirmish" between a BP "company man", a driller and engineers: "The driller was outlining what would be taking place, whereupon the company man stood up and said 'no, we'll be having some changes to that'," Brown said.

A memo given to a congressional committee by BP reveals events as workers prepared to put a cement plug on the well in preparation for the rig to be moved. Two hours before the explosion, tests showed a buildup of pressure – and subsequent decisions to press ahead with the operation are under scrutiny. BP has pointed out that other firms were involved – it was leasing the rig from Transocean, which owned and operated it, while the US firm Halliburton was responsible for a cement plug.

A poll by CBS News found 70% disapproval of BP's handling, and 45% unhappy with the Obama administration's response. Interior secretary Ken Salazar described it as a "massive and potentially unprecedented environmental disaster", and BP would be liable for costs beyond a usual $75m (£52m) maximum liability for oil firms in clean-ups: "BP will be held accountable for costs of the government in responding to the spill and compensation for loss or damages."

As shrimpers, fishermen and tourism industry workers along the Louisiana coast see their livelihoods dwindle, BP has watched its share price slump by 28%, wiping $84bn off its market value. Some 1,200 vessels and 22,000 people are involved in the effort to temper the scale of the disaster.

The US agency overseeing oil companies is also under intense criticism. An official report found that the Minerals Management Service allowed staff at oil and gas firms to fill in inspection reports in pencil, with regulators later going over the answers in ink.

Mary Kendall, acting inspector general at the department of the interior, told a congressional committee yesterday that there were problems with "gift acceptance, fraternising with industry and pornography" at the agency. She suggested there was a problem with the closeness of ties between watchdogs and industry executives: "The individuals involved in the fraternising and gift exchange – both government and industry – have often known one another since childhood."