Investigators in the US say BP had been warned that cement used to seal the Deepwater Horizon well was unstable before it blew out, causing a massive oil leak in the Gulf of Mexico.
The National Oil Spill Commission has been looking into the causes of the explosion that killed 11 workers and led to the largest offshore oil spill in US history.
A White House Panel said Halliburton, which was BP's cementing contractor, used flawed cement when securing the bottom of the well.
But this contrasts with the company's statements claiming its tests showed the mixture was stable.
Halliburton, which has blamed BP's well design and operational decisions for the disaster, acknowledged in a six-page statement released on Thursday that it never tested the final mixture of cement for stability after BP made a last-minute change to the mix.
The cement's failure to prevent oil and gas from entering the well has been identified as one of the causes of the April 20 disaster.
BP and Halliburton decided to use a foam slurry created by injecting nitrogen into cement to secure the bottom of the well, a decision outside experts have criticised.
The investigating panel said three of the four tests carried out by Halliburton in February and April failed to show the mix would hold.
Members of the US coastguard walk past a Halliburton office
The results of the single successful test were not shared with BP before the cement was pumped, according to a letter sent to commissioners by the chief investigative counsel Fred H. Bartlit Jr.
"The fact that BP and Halliburton knew this cement job could fail only solidifies their liability and responsibility for this disaster," said Democrat Edward Markey, who has criticized BP and its well partners.
There are also questions about whether BP pumped down enough cement to seal off the bottom of the well, which was located more than three miles below sea level.
This is the first time any company other than BP has been seen to be responsible for the disaster.
Halliburton's investors will be concerned the firm could be forced to bear some of the clean-up costs.
Sky's US correspondent Robert Nisbet said this new evidence will be "embarrassing" for Halliburton but "not unexpected".
BP has taken a $32.2bn (£20bn) earnings charge to cover the cleanup.
Halliburton was founded in the US in 1919 and provide of products and services to the oil and gas industry.
It employs over 50,000 people in 70 countries