Bhutto died while she was standing inside a car through the open sunroof during a political parade in the city of Rawalpindi in December, six weeks after her return from self-imposed exile. She was running as the main opposition candidate in parliamentary elections now scheduled to take place Feb. 18.
One suicide bomber killed her, the investigation found, contrary to the speculation that two men carried out the attack.
"The inevitable conclusion is that there was one attacker in the immediate vicinity of the vehicle in which Ms. Bhutto was traveling," the report said, according to the Washington Post.
British doctor Nathaniel Cary, who was cited in the report, said X-rays showed that Bhutto died when the blast caused her head to hit the edge of the vehicle's sunroof. He did not rule out that she also may have been hit by gunfire in the neck or upper torso, but he said it was the head injury that was the main cause of death.
Pakistani government officials came to similar conclusions soon after the assassination.
The Scotland Yard report was met sharp condemnation from members of Bhutto's Pakistan Peoples Party, which renewed calls for an independent investigation by the United Nations to find out who perpetrated the attack.
"We don't agree with their views on the cause of death," Sherry Rehman, a spokeswoman for the PPP, told Bloomberg News. "It gives all the more reason to call for international investigation. We are now more interested to know the hands involved in her killing than the cause."
The Pakistani government has said militants with links to al-Qaida plotted the attack.
The results of the investigation were released a day after thousands of Bhutto supporters visited her gravesite to mark the end of a 40-day mourning period.