BP said today that its "static kill" operation to begin permanently shutting off the Gulf of Mexico oil well had been a success.
The embattled oil giant started pumping heavy mud into the well yesterday and said it had now reached a "static condition", with pressure being controlled as hoped.
The news marks the first stage in bringing to an end the worst spill in US history.
The group said it was a "significant milestone", although it will closely monitor the well to see if more mud needs to be pumped in, with aims to add in cement if the pressure remains controlled.
BP temporarily sealed the well after a tightly-fitting cap was placed over it two weeks ago.
This stopped oil gushing into the Gulf for the first time since the leak erupted on April 20, when the Deepwater Horizon oil rig exploded and sank, killing 11 workers.
BP said the so-called static kill will pave the way for permanent closure of the well, which will be achieved using two relief wells.
The group has been drilling the relief wells since soon after the disaster struck.
They are expected to be completed by mid-August, when they will be used to pump further cement and mud into the well in a final phase of the operation.
"A relief well remains the ultimate solution to kill and permanently cement the well," said BP.
The US government estimates that the well leaked 4.9 million barrels of oil before being capped last month.
BP has been attacked in the US over its handling of the affair and last week confirmed under-fire boss Tony Hayward would step down in October, to be replaced by American chief Bob Dudley.
The group also revealed a 32.2 billion US dollar (£20.8 billion) blow from the spill in second-quarter figures.
BP is now facing tough sanctions in the US, with American politicians last week passing a Bill proposing to freeze the firm out of new drilling leases for seven years.
BP spokeswoman Sheila Williams said the mud is holding the oil down.
The static kill - also known as bullheading - involves slowly pumping the mud from a ship down lines running to the top of the ruptured well a mile below. BP has said that may be enough by itself to seal the well.
Workers stopped pumping mud into the well after about eight hours of their "static kill" procedure and are monitoring the oil well to ensure it remains stable, BP said.
"It's a milestone," Ms Williams said. "It's a step toward the killing of the well."
The next step will be deciding whether to cement the well, she added.
To call the entire mission a success, crews working on a flotilla of vessels on a desolate patch of water need to seal off the well from two directions.
An 18,000ft relief well BP has been drilling for the past three months will be used later this month to execute a "bottom kill", in which mud and cement will be injected into the bedrock more than two miles below the sea floor to finish the job, retired Coast Guard Admiral Thad Allen said.
A 75-ton cap placed on the well in July has been keeping the oil bottled up inside over the past three weeks, but is considered only a temporary measure. BP and the coast guard want to plug up the hole with a column of heavy drilling mud and cement to seal it off more securely.
Before the cap was lowered on to the well, 172 million gallons of crude flowed into the sea, unleashed by the April 20 explosion aboard the drilling rig Deepwater Horizon that killed 11 workers.
BP won't know for certain whether the static kill has succeeded until engineers can use the soon-to-be-completed relief well to check their work.
The task is becoming more urgent because peak hurricane season is drawing near, Admiral Allen said.
Tropical Storm Colin formed then dissipated far out in the Atlantic on Tuesday, but early forecasts say it will travel toward the US east coast rather than the Gulf.