Engineers will today begin testing a new sealing cap on BP's gushing wellhead in the Gulf of Mexico as hopes rise of containing the oil spill during the looming hurricane season.

Work on installing the cap using robots a mile below the surface of the sea was successfully completed last night as efforts intensified to capture all the oil that has poured into the sea since the Deepwater Horizon explosion killed 11 people on 12 April.

Tests on the well's internal pressure will take between six and 48 hours to establish whether the flow has been stopped or leaks remain elsewhere. The developments may mark a turning point in efforts to stop the flow until more permanent measures, including the drilling of two relief wells, can be completed by the middle of next month.

BP said: "It is expected, though cannot be assured, that no oil will be released to the ocean for the duration of the test. This will not, however, be an indication that flow from the wellbore has been permanently stopped."

The company added that the sealing system had never before been deployed at such depths or under such conditions. It replaces a previous cap, fitted last month, which managed to contain only about half the escaping oil. Estimates put this as between 35,000-60,000 barrels a day. It is hoped the new cap will be able to siphon the entire flow for collection on the surface.

Former US coastguard Admiral Thad Allen, who is overseeing the US government response operation, said measurements during the test would "provide valuable information about the condition of the well below the sea level and help determine whether or not it is possible to shut the well for a period of time, such as during a hurricane or bad weather, between now and when the relief wells are complete".

He said the tests could continue for more than two days if necessary, adding that "the government will halt the test if the risks of doing further damage to the surrounding formation are significant".

The new cap-and-seal stack is larger than the previous one and is bolted over the top of the wellhead rather than clamped loosely over it. Allen said it was possible the new cap could be used to shut down the wellhead again temporarily "for a period of time, such as during a hurricane or bad weather".

Doug Suttles, BP's chief operating officer of exploration and production, suggested the cap might be used to keep the well closed for a longer stretch. "Depending on the results, we'll either continue to contain the flow while we wait on the relief well or potentially be able to close the flow in," he said.

The news comes as BP revealed the cost of clearing up the environmental and economic disaster in the Gulf and on its damaged shoreline had reached $3,5bn (£2.33bn) and amid a growing public row over Barack Obama's efforts to stop new drilling projects in the Gulf. The White House has outlined a more limited ban following anger at attempts to implement a six-month moratorium. A federal court in Louisiana struck down the proposals, ruling that they were too broad.

Yesterday, the first of two relief wells, begun on 2 May, was about 190ft (58m) from intersecting the blown-out well 13,000ft beneath the seabed, Suttles said. The relief well could reach its target by the end of this month, keeping it on schedule to actually kill the leak by mid-August.

If the relief wells fail, BP could install a new permanent oil-capture system by late August or early September, Kent Wells, senior vice-president of exploration and production, told Obama's commission investigating the spill.