A leak detected in the choke line has delayed the start of the critical BP well integrity test, the British oil giant said.

"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 3 ram stack," BP said in a statement.

"It has been isolated and will be repaired prior to starting the test."

Procedures leading up to the actual well integrity test, which included disconnecting the Q4000 and Helix Producer vessels which were collecting oil from the wellhead, were already under way when the leak issue emerged.

Hopes are high that the well integrity test can proceed and finally close a burst well, stopping more crude from surging into the Gulf of Mexico in what has become the United States' worst environmental disaster.

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.

Admiral Thad Allen, the former US Coast Guard chief leading the government's response to the 85-day disaster, earlier gave a green light to BP to proceed with the test.