BP chiefs say they may be able to stem the Gulf of Mexico oil spill with lines running from a ship to the blown-out well a mile below, after months insisting that a pair of costly relief wells were the only sure way to kill the leak.

As crews planned testing to determine whether to proceed with a “static kill” to pump mud and perhaps cement down the throat of the well, BP senior vice president Kent Wells said if it was successful the relief wells may not be needed after all, to do the same weeks later from the bottom.

The primary relief well, near completion, will still be finished and could be used simply to ensure the leak is plugged, Mr Wells said.

“Even if we were to pump the cement from the top, we will still continue on with the relief well and confirm that the well is dead,” he said. Either way, “we want to end up with cement in the bottom of the hole”.

Before the effort can begin, engineers must probe the broken blowout preventer with an oil-like liquid to decide whether it can handle the static kill process. They had hoped to begin the hours-long test yesterday but delayed it until today after a small leak was discovered in the hydraulic control system.

The US government and company executives have long said the relief wells, which can cost about £63 million each, may be the only way to make certain the oil is contained to its vast undersea reservoir.

Meanwhile, a government task force estimated that about 172 million gallons of oil made it into the Gulf between April and mid-July, when a temporary cap bottled up all the oil. The earlier estimate had been as high as 184 million gallons.

The company began drilling the primary, 18,000ft relief well on May 2, 12 days after the Deepwater Horizon rig exploded and killed 11 workers, and a second back-up well on May 16.

The first well is now only about 100 feet from the target, and Mr Wells said it could reach it as early as August 11.

“Precisely what the relief wells will do remains to be seen given what we learn from the static kill,” BP spokesman Daren Beaudo said. “Can’t predict it for certain.”

Retired Admiral Thad Allen, the US government’s point man on the spill response, said the focus was now on making sure the static kill was successful. But he warned that government officials did not see it as “the end all, be all until we get the relief well done”. The static kill is meant as a bit of insurance for the crews who have spent months fighting the oil spill. BP and government officials have managed to contain large parts of the spill through skimmers, oil-absorbant boom and chemical dispersants meant to break up the oil.

US regulators have come under fire from critics who say BP was allowed to use excessive amounts of the dispersants, but government officials say they have helped cut the use of the chemicals dramatically since late May.

The Environmental Protection Agency released a study yesterday concluding that when mixed with oil, chemical dispersants used to break up the crude in the Gulf were no more toxic to aquatic life than oil alone.

Timeline since disaster struck in April

A summary of events since the disaster struck in April:

April 2010

• 20: The Deepwater Horizon oil rig explodes in the Gulf of Mexico, 80 kilometres off the coast of Louisiana, killing 11 workers.

• 22: The platform sinks to the sea floor, some 1,500 metres below the surface.

• 25: Energy giant BP, which leased the rig, says oil is escaping from the well, after initial BP denials of leaks.

• 30: Oil starts washing ashore in Louisiana, eventually soiling more than 640 miles of coastline across five US states.

May 2010

• 2: President Barack Obama visits Louisiana.

• 27: Obama unveils six-month moratorium on new offshore oil drilling and exploration.

• 30: BP chief executive officer Tony Hayward says he “would like my life back.”

June 2010

• 1: US launches a civil and criminal investigation into the spill.

• 3: After a series of failures, BP moves to place a containment cap securely over severed pipe.

• 15: Obama vows to “make BP pay”. US raises estimate of the leak flow rate to between 35,000 and 60,000 barrels a day.

• 16: BP announces a $20-billion fund to compensate people affected by the disaster.

• 22: A New Orleans judge blocks the deep water drilling freeze.

• 30: Hurricane Alex disrupts containment efforts for several days.

July 2010

• 5: BP spill costs soar above three billion dollars.

• 12: BP says it successfully placed a new giant cap over the leak and will start integrity tests.

• 15: No oil is flowing into Gulf, says BP.

• 27: BP boss Hayward resigns, replaced by American Bob Dudley.

August 2010

• 2: US government and BP release revised spill figures, saying that a total of 4.9 million gallons of crude gushed from the ruptured well, making it the world’s biggest-ever accidental oil spill.

• 3: BP was expected to begin its “static kill” operation to permanently seal the well with heavy drilling mud and cement.