Video footage showed a gunman firing toward her vehicle as she left the rally, following which an explosion took place.
Martin Nesirky, a UN spokesman, said on Wednesday that the panel headed by Heraldo Munoz, Chile's ambassador to the UN, would formally present its report to UN chief Ban Ki-moon on Thursday.
Ban will then share it with the Security Council and with the Pakistan ambassador to the UN, Nesirky said.
Munoz and one of the other panel members, Marzuki Darusman, an Indonesian ex-attorney general, were to give a press conference later on Thursday to provide details of the report.
Kristen Saloomey, Al Jazeera's correspondent at the UN, said: "It's very unusal that we haven't seen a glimpse of this report yet. Most UN reports are leaked to the media ahead of time, so it's a sign of how sensitive this one is.
"While there will likely be no "smoking gun", and no names named in this report, it could have some serious repercussions for the government of Pakistan.
"In particular, a current member of the government - interior minister Rehman Malik - who was Bhutto's chief of security, is expected to come out badly after this report is released.
"Equally criticised will likely be how the crime scene was handled. Just an hour after Benazir Bhutto's body was removed from the scene, the crime scene was washed down, destroying any possible forensic evidence."
The delay in releasing the report was announced late last month only hours after a UN spokeswoman in Islamabad said all UN offices in Pakistan would close for three days as a security precaution.
The measure affected more than 2,000 staff in dozens of offices around the country.
Qamar Zaman Kaira, Pakistan's information minister, who announced the delay, said that two unnamed officials who had warned Bhutto that her life might be in danger "could be helpful to the commission in finding who was behind her assassination".
Tariq Pirzada, a political analyst and columnist in Islamabad, told Al Jazeera: "It has been widely orchestrated as a report that will be impartial. The government of Pakistan invested a lot of time and money in this report.
"But the attempt to discredit the report by the Zardari government in requesting for its delay has a reason. Maybe the report strongly implicates all. Maybe the report suggests that the personal security provided by Mr Rehman Malik and his 400 guards was not sufficiently managed and handled.
"This would jeopardise the credibility of the current government."
Security is precarious in parts of Pakistan, where more than 3,150 people have been killed in suicide and bomb attacks over the last three years.
Bhutto wrote in her autobiography of warnings that four suicide squads - including one sent by a son of Osama bin Laden, the al-Qaeda leader - were after her.
She also repeatedly accused a cabal of senior Pakistani intelligence and government officials of plotting to kill her, notably in an attack that killed 139 people in Karachi on October 18, 2007 when she returned from exile.