Oil giant BP detected a leak Thursday in equipment to seal the catastrophic Gulf of Mexico gusher, forcing another delay to the start of crucial tests on a new tighter-fitting cap.
After finally getting the green light to begin pressure tests, BP said it would have to postpone the procedure for a second time to repair a leak in a so-called choke line, which leads off from a system of blowout preventers being used in the operation.
"In preparation for commencement of the well integrity test, the middle ram has been closed and a leak has been detected in the choke line of the three-ram stack," BP said in a statement on its website.
"It has been isolated and will be repaired prior to starting the test."
A blowout preventer is a large device with a series of valves, also referred to as rams, which are placed at the top of a well that can be closed for safety reasons during drilling.
The blowout preventer used in the damaged BP well contains elements of three types of valve, two of which are capable of sealing pipes of various diameter while a third seals the wellbore itself.
It was not immediately clear for how long the leak would delay the tests but it represents another blow to the marathon efforts to put an end to what is already the worst environmental disaster in United States' history.
On Wednesday, the former US Coast Guard chief leading the government's response to the 85-day disaster gave the British oil giant approval to begin the tests after overcoming fears they could lead to "irreversible leakage" below the seabed.
"At this time we will be releasing an order to BP to proceed with the well integrity test," said Admiral Thad Allen.
"This test will run for a maximum of 48 hours at which time we will stand down, assess where we're at, and assess the next steps."
Allen on Tuesday delayed the high-stakes procedure for 24 hours for further analysis to be conducted, and only made the decision to proceed after meetings with top oil industry experts and US officials, including Energy Secretary Steven Chu, a Nobel Prize-winning physicist.
Procedures leading up to the actual test, which included disconnecting the Q4000 and Helix Producer vessels which were collecting oil from the wellhead, were already under way by the time the latest delay was announced.
The test involves shutting off the valves on a 75-tonne cap installed on Monday to evaluate the integrity of the well bore, which stretches down 2.5 miles (four kilometers) below the seabed.
High pressure readings would allow the three valves to remain shut and the well would effectively be sealed, but low ones could mean there is a hole somewhere in the casing of the well where oil is escaping.
"It will be terrific news if we can shut in the well but I don't think we can say that," Allen said. "I don't want to get anybody's hopes up that we can shut this well in until we get the empirical readings we need."
The White House said President Barack Obama was being kept up-to-date.
Chu will be involved in the consultations that will take place every six hours during the integrity test, alongside BP and government scientists, on whether to continue the procedure, said BP senior vice president Kent Wells.
If the tests are successful, officials say the new cap could give them the ability to capture all the leaking oil in a matter of days.
The Gulf disaster has cost BP some 3.5 billion dollars (2.78 billion euros) and compensation could mean it ends up forking out 10 times that figure.
BP shares were at 400.45 pence in early London trading on Thursday, down 0.14 percent.
The new delay added to the agony for Gulf residents whose livelihoods depend on the oil being stopped and the huge surface mess being cleared up.
Once the valves on the cap are closed, no oil will be streaming into the Gulf for the first time since the Deepwater Horizon rig in April.
Drilling on a relief well that is now only four feet (1.2 meters) away from the leaking well will be suspended when the integrity test starts.
Even if the well is found to be secure and the cap is left on, the relief well will be drilled to completion so the oil reservoir can be permanently plugged with cement in a "kill" operation expected in mid-August.
Explore further: BP told to use less toxic dispersant in oil spill: report