BP has been accused of hiding important data needed to properly investigate the Gulf of Mexico oil spill.

Transocean is the company that owns the rig, and they alleged that BP refused to give over information about the initial explosion.

The claim was made in a letter from a Transocean lawyer and sent to President Obama's cabinet.

BP rejected the claim, saying the letter had "misguided and misleading assertions".

The claim risks further pressure adding up on BP, which has been the brunt of policital and public criticism since the disaster on April 20 killed 11 workers and caused the worst oil spill in US history.

Steven L. Roberts, a lawyer for Transocean, said in the letter, "BP has continued to demonstrate its unwillingness, if not outright refusal, to deliver even the most basic information to Transocean.

"This is troubling, both in light of BP's frequently stated public commitment to openness and a fair investigation and because it appears that BP is withholding evidence in an attempt to prevent any other entity other than BP from investigating," he wrote.

According to the AFP news agency, the letter was sent to members of Obama's team and Congressional leaders.

Transocean is looking at 249 lawsuits for damages over the disaster. The company has asked the court to limit its liabilities to $27 million, claiming it was not responsible.

A scientific inquiry into the aftermath of the spill has confirmed that the continued presence of a tocis chemical residue one kilometer below the ocean surface.

The investigation is being carried out by the Woods Hole Oceanographic Institute. It shows a plume of crude oil based chemicals up to 200m high and 2km wide, extending 35km from the site of the spill.

BP said in a statement that it was "unequivocal and steadfast" in its commitment to find out why the Deepwater Horizon rig exploded.

"We are disappointed that Transocean has opted to write a letter with so many misguided and misleading assertions, including the assertion that BP is 'withholding evidence'" on the explosion and spill.

"We have been at the forefront of co-operating with various investigations commissioned by the US government and others into the causes of the Deepwater Horizon tragedy," BP said.