Calling for drastic changes, the investigation by an independent UN-appointed panel said UN security officials ignored warning signs and dismissed protection from the US military in Iraq without finding a substitute.
The report blames security officials in New York and Iraq as well as top management for lapses before the August attack in Baghdad that killed 22 people, including chief of the UN mission Sergio Vieira de Mello. An estimated 150 people were injured.
But it singles out no specific individuals, a point noted by the UN staff union, which called on the UN General Assembly to take up the issue.
"The main conclusion of the panel is that the current security management system is dysfunctional," said the 40-page report by former Finnish President Martti Ahtisaari, appointed by UN Secretary-General Kofi Annan to lead the investigation.
Ahtisaari, who says there is little
accountability for UN security
Uneasy about a heavy US military presence, UN officials asked occupation authorities to withdraw heavy equipment from the compound in the Canal Hotel, dismantle an observation post on the roof and remove obstacles from the access road - the road used by the driver of the flatbed truck carrying the explosives.
"Adequate security arrangements may not have been able to prevent the attack against the Canal Hotel perimeter, but would certainly have minimised the vulnerability of the staff and premises and reduced the number of casualties caused by the attack," the report said.
Annan will study the report closely and will take steps "to ensure early implementation of its main recommendations," UN spokeswoman Hua Jiang said.
Like the UN system itself, Ahtisaari said security was scattered among semi-independent UN relief agencies and political staff.
He said there was little accountability, no clear chain of command, a stifling bureaucracy, too little money, and too few professional staff to evaluate intelligence.
The deficiencies cited by Ahtisaari's team ran the gamut of inadequecies - from not knowing the number of foreign staff in Baghdad, to delaying, even until the present day, the installing of shatter-proof glass, to not reacting to reports shortly before 19 August stating that the UN offices were a target.
UN special envoy Sergio Vieira de
Mello (L) was killed in the attack
The security personnel, however, did make recommendations.
They were ignored - even by Annan, who was under pressure to keep staff in Iraq and who reduced them only gradually until today only a handful remain.
The report also points to a steering group on Iraq, headed by Deputy Secretary General Louise Frechette, saying the body did not have a proper chain of command needed for security.
At the same time, Ahtisaari told a news conference that the UN had failed to recognise that it was associated with a decade of drastic sanctions in Iraq and with the world's most intrusive arms inspections.
Recalling his days as head of the UN mission in Namibia between 1989 and 1990, Ahtisaari described that venture as "like a Sunday school picnic compared to present operations".