"BP has mismanaged this entire incident from day one. They should not be trusted," he said.
BP denied any cover-up and said some third-party estimates of the leak were inaccurate.
BP spokesmen said the original leak estimate came from the US national oceanic and atmospheric administration, one of its federal partners in the joint spill response.
The oil giant said damage to the end of the pipe had distorted its diameter by about 30 per cent, while a drill pipe trapped inside the riser had reduced the flow area by an additional 10 per cent, "thus, some third party estimates of flow, which assume a 19.5 inch diameter, are inaccurate."
"I understand the frustration," Doug Suttles, BP's chief operating officer, told CBS news channel. "We're supplying information."
BP said its engineers were working with US government scientists to determine the real size of the leak, and was in the process of providing the US government's Flow Rate Technical Team with all requested information.
Adding to the confusion, BP on Friday revised downward an estimate from Thursday that one of its containment solutions, a 1.6km long siphon tube inserted into the larger of two seabed leaks, was capturing 5,000 barrels 795,000 litres of oil per day.
John Curry, a BP spokesman, said the amount of crude oil it sucked from the leak fell to 2,200 barrels (350,000 litres) a day in the 24-hour period ended at midnight on Thursday.
"The flow changes. It's not constant," Curry said.
BP is still trying to stop the leak that threatens environmental and economic catastrophe for the US Gulf coast, with its next planned step being to try a "top kill", whereby heavy fluids and then cement will be pumped into the gushing well to try to plug it.
This is not expected to take place until Sunday or possibly Tuesday.
A month after the well blowout and rig explosion that unleashed the catastrophic spill, sheets of oil are starting to clog fragile marshlands on the fringes of the Mississippi Delta, damaging fishing grounds and wildlife.
Scientists fear that parts of the huge fragmented surface slick will be sucked to the Florida Keys and Cuba by ocean currents.
On the Louisiana coast, fishermen counted the cost to their livelihoods.
"This is going to keep killing stuff and it will make whole areas incapable of supporting marine life," George Barisich, president of the united commercial fishermen's association, said.
John Terrett, Al Jazeera's correspondent in Venice, Louisiana, said people there can cope with what they have seen so far but [with the oil spill still not stopped] they are worrying about what has yet to come.
"We know it's coming ashore to the east of us here, and to the west, to Grand Isle, where there are some oyster beds.
"So far none of them have been contaminated, but oysters are one of the great Louisiana exports [so people are worried]," Terrett said.
BP has promised to pay legitimate damages claims and faces billions of dollars in expected cleanup and damages costs.
Shares in BP were down 3.4 percent at 511 pence by 1202 GMT, having earlier struck a nine-month low due to the cover-up allegations