The White House has rejected criticism of the way it handled the Gulf of Mexico oil spill from the commission investigating it.
The Obama administration blocked government scientists' efforts to inform the public of worst case scenarios, a draft report said.
But spokesman Robert Gibbs said the White House had not tried to block "the most accurate and timely information".
The commission was appointed by President Barack Obama.
The explosion on the Deepwater Horizon drilling rig on 20 April killed 11 workers, and led to the pollution of hundreds of miles of shoreline and disruption of tourism and fishing before the leaking well was capped on 15 July.
In preliminary findings released on Wednesday, the National Oil Spill Commission said officials were too optimistic about handling the disaster, one of the worst in US history.
In a briefing for journalists on Thursday, Mr Gibbs said the draft "has not necessarily been signed off by members of the commission".
The report suggests the White House was directly involved in controlling information from the spill.
The report says that during the crucial first 10 days of the oil spill the government's response "seemed to lag" - and that coastguard officials were "overly-optimistic" in believing BP could handle the incident.
Government scientists are accused of first underestimating the amount of oil pouring into the Gulf and then overestimating the quantity which had evaporated or otherwise been removed from the sea.
The report says the president's adviser on energy policy, Carol Browner, stated wrongly on American television that three-quarters of the oil had "gone", and that the White House blocked a request by officials to make public a worst-case scenario for the scale of the leak.
The panel, which cited interviews with government officials, said that in late April or early May, the White House budget office had denied a request from the National Oceanic and Atmospheric Administration (NOAA) to make public its worst-case estimate of how much oil could leak from the Macondo well.
But the White House said in a statement that Interior Secretary Ken Salazar and Thad Allen, the retired Coast Guard admiral in charge of the response, had told the public the worst-case scenario could be more than 4.2 million gallons per day.
BP's drilling permit for the well said 6.8 million gallons could pour from the well during the worst-case scenario.
NOAA and the Coast Guard later received an updated worst-case scenario just after the spill had begun, putting the estimate at 2.7 million to 4.6 million gallons per day.
The National Oil Spill Commission report says those numbers, which were used as the basis for the administration's spill response, were never made public - though they appeared on an internal Coast Guard situation report.
The report also says the federal government used too much boom to stop the oil from spreading - especially in the US state of Louisiana, where directions went out to "keep the parishes happy".