A leak in a crucial piece of undersea equipment forced oil giant BP to interrupt and briefly postpone the test that may determine whether its leaking gulf well can be sealed with a new cap, pushing back the collection of any meaningful data until later Thursday, a company official said.

As the company began the process of shutting off a series of valves on the equipment that sits atop the well Wednesday evening, crews noticed a leak on one of the lines, called a choke line, that diverts the oil.

The company disconnected the faulty system and replaced it with a new one, BP Senior Vice President Kent Wells said in a Thursday morning conference call with reporters.

"Now we just need to go forward and retest it to make sure this one's fine, and then we'll go forward with the test," Wells said.

The test, which requires temporarily sealing off the well at the top, will gauge the pressure inside. If high pressure is measured for a sustained period of time -- at least 48 hours -- it could indicate that the well is not leaking, and that it may be safe to keep it sealed.

Low pressure may indicate that the well is leaking oil from other locations. If that is the case, the cap could exacerbate the flow and, in the worst-case scenario, push oil to the sea floor, creating new surface leaks. BP officials said that if they detect low pressure, they will unseal the cap and revert to an oil collection plan, using a series of pipes leading to containment ships on the surface. That plan would collect some of the oil at first, but the company promises it will have the capacity to collect all of the leaking oil -- up to 60,000 barrels per day -- by the end of July.Gulf oil spill: BP postpones test for sealing off well when leak in equipment is detected

-- Richard Fausset in Atlanta