Diplomatic and political sources say the report is expected to implicate Syrian and Lebanese officials.
UN Secretary-General Kofi Annan planned to transmit the report he received on Thursday from veteran German prosecutor Detlev Mehlis to the 15-nation Security Council and to the Lebanese government on Friday.
The United States is seeking tough action against Syria, and Western powers are already discussing their response.
Mehlis had no plan to give an advance copy to Damascus, UN chief spokesman Stephane Dujarric said.
Syrians have grown increasingly nervous over how strongly the report will implicate their officials in the assassination of al-Hariri and 20 others in a Beirut truck bombing on 14 February.
Syrian President Bashar al-Assad insisted last week that his country was "100% innocent".
But the German news magazine Stern said this week that Mehlis would name al-Assad's brother-in-law, Syrian military intelligence chief Asef Shawkat, as a suspect in the deadly blast.
Shawkat is widely seen as the second most powerful man in Syria after al-Assad. For Mehlis to point the finger at a member of al-Assad's inner circle would be political dynamite.
Annan warned reporters on Wednesday against seeking to exaggerate or politicise the report ahead of its release.
"Mehlis's report is the beginning, not the end, because the magistrates and others will have to follow through on that report and decide whom to charge and whom to bring to the dock," the UN leader said.
Many Lebanese see Syria's hand
in the assassination of al-Hariri
The US, France and Britain have been discussing two possible UN resolutions as follow-ups to the report. No texts have been drawn up, although each party has a wish list, diplomats said.
Syria and Lebanon were an important focus during US Secretary of State Condoleezza Rice's visit last week to London, Paris and Moscow, where she tried to rally support for punitive action against Damascus.
US Ambassador John Bolton said Washington planned to study the report carefully before responding.
"We have been in consultations and discussions with a number of other countries on what the contingencies might be and what our reaction would be," he said.
Lebanon has already arrested four pro-Syrian Lebanese security chiefs and charged them with murder on Mehlis's recommendation. Diplomats and political sources in Lebanon said Mehlis will also name Syrian officials in his report.
Last week, Syrian Interior Minister Ghazi Kanaan was found dead in his office, his apparent suicide coming three weeks after he was questioned by the Mehlis's team.
Judicial sources in Beirut said this week that a key Syrian witness detained in France had been charged with murder in connection with al-Hariri's death.
Lebanon has been in a state of
alert, awaiting the Mehlis report
French police detained Mohammed Zuhair al-Siddiq on Sunday on an international warrant because they believed he had an indirect role in al-Hariri's killing and had misled international investigators, the sources said.
Syrian Deputy Foreign Minister Walid al-Mualim said France
and the US would use the report to launch a phased plan to isolate Syria and impose economic sanctions on it.
"The first stage consists in influencing Arab countries so that we cut our relations with Iraq, Palestine and Lebanon. We are now at the second stage, which aims to isolate us," al-Mualim told France's Le Figaro in an interview published on Thursday.
"The next one will be to impose economic sanctions via a UN resolution. But we think that the Russians and the Chinese will oppose these sanctions," al-Mualim said.
The Security Council will discuss the report next week and consider its response. The council will also consider a Lebanese request for continued UN help to bring to trial the suspects in the assassination.
The Beirut government wants Mehlis, who began work in June, to extend his mission to mid-December and says it would prefer an international court to handle the trial of any suspects.