BP bears ultimate responsibility for the worst offshore oil spill in U.S. history, a key government panel said Wednesday in a report that assigns more blame to the company than other investigations and could hurt its effort to fend off criminal charges and billions of dollars in penalties.

The report concluded that BP violated federal regulations, ignored crucial warnings, was inattentive to safety and made bad decisions during the cementing of the well a mile beneath the Gulf of Mexico.

Eleven rig workers were killed in the April 2010 explosion, and some 200 million gallons of crude spewed from the bottom of the sea.

The investigation was conducted by a team from the two main agencies responsible for drilling and safety in federal waters: the Coast Guard and the Bureau of Ocean Energy Management Regulation and Enforcement.

In the report, other companies shared some of the blame. Rig owner Transocean was accused of being deficient in preventing or limiting the disaster, in part by bypassing alarms and automatic shutdown systems. Halliburton, the contractor responsible for mixing and testing the cement, was faulted as well.

But BP, as the designated operator of the Macondo well, “was ultimately responsible for conducting operations at Macondo in a way that ensured the safety and protection of personnel, equipment, natural resources and the environment,” the panel concluded.

The report identifies many of the same causes and faulty decisions found by previous investigations, including those conducted by a presidential commission, congressional committees and the companies themselves. But some of those earlier investigations spread the blame more evenly.

The new report also marks the first time an investigative body looking into the spill has identified specific violations of federal regulations by BP and its contractors.

The findings will be used to shape reforms in offshore drilling safety and regulation. They will also be used by lawyers for victims involved in court battles over the oil spill, and by government agencies considering charges and penalties.

“It is only a question of time before BP — along with Transocean and Halliburton — will face criminal charges for their roles in the Gulf oil spill,” said David Uhlmann, a University of Michigan law professor who formerly led the Justice Department’s environmental crimes section.