The report called for a full investigation of the bomb on February 14 by an “international independent commission” but said that the top tier of Lebanon’s security services must first be replaced.

“It is doubtful that such an international commission could carry out its tasks satisfactorily — and receives the necessary active cooperation from local authorities — while the current leadership of the Lebanese security services remains in office,” it said.

The UN team, led by Peter Fitzgerald, Ireland’s deputy police commissioner, concluded that Mr Hariri was killed by about 1,000kg of TNT planted in a heavy vehicle, which exploded when his motorcade drove past the Hotel St Georges in Beirut.

The team said that it had received accounts of a final ten-minute meeting in Damascus last year at which Mr Hariri hoped to convince the Syrian leader not to back a three-year extension of Emil Lahoud’s term as Lebanese President. Mr Assad, who refused to meet the UN team, reportedly told Mr Hariri that Mr Lahoud should be viewed as his personal representative in Lebanon and “opposing him is tantamount to opposing Assad himself”. Mr Assad then added that he “would rather break Lebanon over the heads of Hariri and (Druze leader Walid) Jumblatt than see his word in Lebanon broken”.

“Mr Assad then threatened both Mr Hariri and Mr Jumblatt with physical harm if they opposed the extension for Mr Lahoud,” the report said.

Despite this threat, the UN team shied away from blaming the assassination directly on Syria, which has kept troops in its smaller neighbour since the Lebanese civil war of 1975-90.

“Clearly Mr Hariri’s assassination took place (with) the backdrop of his power struggle with Syria, regardless of who carried out the assassination and with what aim,” the team said. “It is nonetheless important to keep in mind that only a proper investigation — not political analysis — could lead to the identification of those who ordered, planned and carried out this heinous crime.”

Nevertheless the team delivered a blistering attack on the role of the Lebanese security services and Syrian military intelligence, saying that they bore the “primary responsibility for the lack of security, protection, law and order in Lebanon”.

It said that Syria wielded influence over its neighbour in “heavy-handed and inflexible manner”. “The Government of Syria bears primary responsibility for the political tension that preceded the assassination of former Prime Minister Hariri,” the report said. “The Government of Syria clearly exerted influence that goes beyond the reasonable exercise of cooperation or neighbourly relations.”

The UN mission found “serious flaws” in the investigation by Lebanese authorities. Lebanese military, police and intelligence personnel, including explosives experts, “interfered with and removed items of possible evidential value”, it said. They also allowed the bomb crater to flood with water from fractured pipes, destroying vital evidence.

The UN team noted the suspicious presence of a white pick-up truck filmed by a security camera at the nearby HSBC bank. But said that this lead had been compromised by investigators. “Parts of a pickup truck were brought to the scene by members of the security services some time after the incident and were placed in the crater and were subsequently photographed and labelled as evidence,” it said.

The UN team also questioned the claim of responsibility by a Saudi-born man of Palestinian origin named Ahmad Abu Adas on behalf of the group “Nasra and Jihad Group of Greater Syria”, broadcast to the al-Jazeera TV station.

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