WASHINGTON — Weeks before the Deepwater Horizon exploded and sank, oil company BP and subcontractor Halliburton learned that tests had shown the cement mixture designed to seal the well was unstable, but they continued to use it anyway, President Barack Obama’s special commission investigating the environmental disaster reported Thursday.

The cement mixture is used to secure the metal casing around the string of pipes and the drill bit as they penetrate the reservoirs of oil and gas lying deep beneath the ocean floor. Cement also is supposed to keep oil and gas from pushing back up the well, causing a blowout.

After running independent tests on the particular cement mix Halliburton used and reviewing the company’s documents, the commission’s chief counsel sent a letter to commission members saying that BP and Halliburton had information about problems with the cement mix they were using in the well but did nothing about it.

“”Halliburton and BP both had results in March showing that a very similar foam slurry design to the one actually pumped at the Macondo well would be unstable, but neither acted upon that data,”” the letter said.

The explosion that sank the oil rig, killed 11 members of its crew and sent millions of gallons of oil spewing into the Gulf of Mexico has become the subject of a Justice Department investigation and numerous lawsuits and other legal actions seeking to hold BP, Halliburton and others liable.

The finding by investigators for the presidential commission is expected to provide fresh ammunition for the companies’ critics and could increase the likelihood of action by the Justice Department.