The report follows a nine-month inquiry by a three-person UN panel and was due to be presented on Tuesday to UN Secretary-General Ban Ki-moon.

But it has been delayed until April 15 because of a request by Pakistani President Asif Ali Zardari, Bhutto's widower, the United Nations said on Tuesday.

"We requested them to include the views of three countries which had warned (Bhutto) after her return that she should take extra precaution because they had information she would be assassinated," said presidential spokesman Farhatullah Babar.

He declined to identify the three countries he said had warned Bhutto.

Bhutto was killed in a gun and suicide bomb attack after an election rally in the city of Rawalpindi on Dec. 27, 2007, weeks after she came back from eight years in self-imposed exile.

The then government led by allies of then president Pervez Musharraf blamed then Pakistani Taleban leader and Al-Qaeda ally Baitullah Mehsud for Bhutto's murder.

Mehsud was killed in a US drone strike last August.

Despite the accusations against Mehsud, conspiracy theories have abounded in Pakistan over who was behind the assassination.

Bhutto was mistrusted by sections of the Pakistani military and security establishment and speculation has lingered that she was the victim of a plot by allies of Musharraf who did not want her to come to power.

A staunch opponent of Islamist militants, Bhutto survived a bomb attack on a rally hours after arriving home in the city of Karachi in October 2007. About 140 people were killed.

Bhutto had returned home to contest an election under a power-sharing deal with Musharraf that the United States had helped broker.

After the Karachi bombing, she had spoken of a warning from a "friendly country" but she did not identify the country.

The UN chief set up the panel in July at the request of Pakistan's coalition government, led by Bhutto's Pakistan People's Party.

The panel, headed by Chile's UN Ambassador Heraldo Munoz, looked into the circumstances surrounding the assassination and its release is not expected to destabilise Zardari's government.

The panel, whose original six-month mandate had already been extended by three months because of the scale of its task, is not expected to name suspected culprits.

Any criminal investigation will be up to Pakistani authorities, but Munoz has said the commission's findings could complement government efforts.

Pakistani Information Minister Qamar Zaman Kaira told Reuters the delay in the submission of the report had been requested so that the panel could get input from two heads of state who he said had warned Bhutto.

He declined to identify them. A UN spokesman told reporters in New York on Tuesday the secretary-general has accepted Zardari's request for a delay but suggested there would be no change to the report.

"My understanding is, and the guidance that I have is, that this is complete, it's done, it's not to be added to," the spokesman said.

Neither Ban nor the Pakistani government had seen the report, the spokesman said.