New Orleans, Louisiana: Oil giant BP detected a leak on Thursday in equipment used 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 system of triple blowout preventers, known as rams.

"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."

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 underway 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 2.5 miles (four kilometres) 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’s a hole somewhere in the casing of the well, from which 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."

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.