The head of the UN team, Peter Fitzgerald of Ireland, informed Lebanese President Emile Lahud that he would leave Beirut on Wednesday and travel to New York, where he would submit his report to UN Secretary-General Kofi Annan, the official news agency ANI said.
The United Nations said in New York that Annan would release the report's findings to the Security Council next week, although no date has been stipulated.
The UN team, made up of Irish, Egyptian and Moroccan investigators and joined by Swiss bomb experts, has been in Beirut since 24 February, gathering information on the killing. Fitzgerald is due to arrive in New York on Thursday.
British daily The Independent said on Monday that Lebanese and Syrian authorities were behind a cover-up of evidence from the blast.
"The UN investigators have become convinced that there was a cover-up of evidence at the very highest levels of the Lebanese and Syrian intelligence authorities," the newspaper said.
"The UN investigators have become convinced that there was a cover-up of evidence at the very highest levels of the Lebanese and Syrian intelligence authorities"
Veteran Middle East correspondent Robert Fisk, based in Beirut, quoted no sources for his report but said members of al-Hariri's family had been told the UN report on the death would be "devastating".
The Lebanese opposition has blamed the explosion on the pro-Syrian regime and Damascus, and launched a campaign of massive public protests against Syria's military presence.
Beirut and Damascus have denied any responsibility for the attack.
Lebanon agreed to cooperate with the UN commission of inquiry, but rejected a full international investigation.
Lahud on Tuesday reiterated his "determination to uncover the perpetrators and those who ordered" the attack.