BP last night delayed a critical test to determine if a new cap on its blown-out Macondo well can arrest the flow of oil that has gushed into the Gulf of Mexico for the past 12 weeks. "We decided that the process may benefit from additional analysis that will be performed [yesterday and today]," said retired Coast Guard Admiral Thad Allen, overseeing the U.S. response to the spill.

Regardless of the results, BP should be able to contain the flow with oil-siphoning vessels by mid-July, he said. As the oil giant prepared for a potential turning point in the worst offshore spill in U.S. history, it also said its plans to sell some non-core assets, which will help pay for a $20-billion clean-up fund, were moving forward. Both pieces of news had helped BP shares maintain their recent recovery in London, although they see-sawed in New York with profit-taking erasing most of the early gains.

"We are in discussions with a number of companies about a number of assets," BP spokeswoman Sheila Williams said in London, declining to give details. "Talks are going well." In Dubai, Abu Dhabi Crown Prince Sheikh Mohammed bin Zayed al-Nahayan said the emirate was considering investing in BP. BP's leaking wellhead is a 1.5 kilometres underwater. The new 73-tonne capping stack was custom-designed and built for the leaking well.

Even if the new cap fails, BP will have the ability to siphon up to 80,0000 barrels per day

of oil from the blown-out well by mid-July and either burn it or store it aboard vessels on the surface, Adm. Allen said.

"Either through a potential shut-in of the well or being able to produce most if not all of the flow that is generated, either way, we will have a way to contain the oil," he said.

Kent Wells, senior vice-president of exploration and production, said BP and government scientists would monitor the cap during the tests at "very minute intervals."

If tests progress as hoped, BP said no oil would flow from the well for the first time since a rig being drilled for BP by Transocean Ltd. sank days after an explosion on April 20 that killed 11 workers. During the tests, two smaller siphoning systems, including the one brought online on Monday, will be turned off. But BP warned the outcome was uncertain since the system has never been tested at such depths.

Depending on the results, BP will either keep the cap closed entirely or use it to resume siphoning oil to ships on the surface. If it works effectively, the cap should either hold all the oil in or allow it to be safely captured and funneled away.

While the cap could contain the flow, Mr. Wells said relief wells remain the sole means to kill the leak permanently. The first of those wells has another nine metres to drill it can intercept the blown-out well by the end of July.