BP is carrying out the final tests on its so-called "top-kill" plan to choke off an oil leak described as the worst in US history.
The energy giant's chief executive said the company would not make a final decision until later on Wednesday on whether to go ahead with the manoeuvre, which has been given a 60 to 70 per cent success rate.
It had been hoped that BP would begin the procedure, which involves smothering the leak with heavy drilling mud and cement, earlier in the day.
Such a method has never been tried out 1,500 metres underwater.
Patience is running out and anger has been growing over the oil giant's inability to stop the oil leak that sprang more than a month ago after an offshore drilling rig exploded on April 20.
Eleven workers were killed in the blast and by the most conservative estimate, 26.5 million litres of crude have spilled into the Gulf of Mexico, fouling Louisiana's marshes and coating birds and other wildlife.
Leak could worsen
But the top kill procedure could also make the leak worse.
Engineers were doing at least 12 hours of diagnostic tests on Tuesday, checking five spots on the well's crippled five-storey blowout preventer to make sure it can withstand the heavy force of the mud.
A weak spot in the device could blow under the pressure, causing a brand new leak. The mud could also tear a new hole in the leaking well pipe.
BP has been drafting plans for the top kill for weeks but had to delay it several times as crews scrambled to assemble the equipment at the site 80km off the coast.
A flotilla of rigs, barges and other heavy machinery stood ready there on Tuesday with a stockpile of some 50,000 barrels of the heavy mud, a manufactured substance that resembles clay.
Tony Hayward, BP's chief executive, said chances of the top kill succeeding were 60 to 70 per cent, while Kent Wells, a BP senior vice-president, said the procedure could be delayed or scuttled if Tuesday's pressure readings were poor.
Wells said it could take anywhere from a few hours to two days to determine whether the top kill was working.
If it succeeds, BP plans to inject a stream of cement to permanently seal up the well and may also install a new blowout preventer.
BP has had limited success with a mile-long tube it installed more than a week ago to siphon up some of the oil, capturing more than 1.9 million litres but also allowing untold amounts to escape into the sea.
Once the results of Tuesday's diagnostic tests are in, BP said, it will consult with government officials, including scientists with the Minerals Management Service (MMS), before deciding whether to press on with the top kill.
But the MMS has been accused of having too cosy a relationship with the very oil companies it is supposed to have oversight of.
Ken Salazar, the US interior secretary, on Tuesday ordered an investigation into whether the rig involved in the massive oil spill in the Gulf of Mexico was properly monitored by the MMS.
The investigation follows a report citing workers from the MMS – which is under the interior department - accepting gifts and possibly allowing oil workers to fill out their own inspection reports.
|An area slightly smaller than the size of Greece is now closed to fishing [GALLO/GETTY]
The report found it was commonplace before 2007 for MMS employees at a Lake Charles, Louisiana office to receive gifts including sporting event tickets and hunting trips from energy companies.
"This deeply disturbing report is further evidence of the cosy relationship between some elements of MMS and the oil and gas industry," Salazar said in a statement.
Salazar, whose interior department has been criticised in recent weeks for not doing enough to prevent the oil rig explosion, said he had directed the agency's acting inspector-general, Mary Kendall, to look into whether MMS employees adequately inspected and enforced standards on the Deepwater Horizon rig.
He also asked the inspector to determine if the improper behaviour outlined in the report had continued since he took office at the department.
The report said a confidential industry source accused some MMS inspectors of allowing energy company workers to fill out their own inspection forms for their platforms, but investigators have so far not been able to determine if any of the files they reviewed were fraudulent.
An inspector-general report released in 2008 found that MMS employees at another office received gifts, as well as used illegal drugs and had sex with workers from the oil companies they were supposed to oversee.
In response to the scandal then, Salazar instituted new ethics rules after he took the helm of the department in 2009.
Salazar and Kendall are set to testify before the House of Representatives' Natural Resource committee on Wednesday on the oil spill.
Barack Obama, the US president who on Friday is set to tour the oil-affected region for a second time, is expected to announce tougher safety requirements for off-shore oil drilling on Thursday, an administration official said on Tuesday.
Also on Tuesday, US officials expanded a fishing ban in the Gulf of Mexico by more than 20,000 square kilometres amid the spreading oil slick.
Some 140,000 square kilometres of water – an area slightly smaller than the size of Greece - are now closed to fishing, the National Oceanic and Atmospheric Administration said, but added that 77 per cent of the Gulf remains open.