A report sponsored by the EU has said an unjustified Georgian assault on breakaway South Ossetia started last year's five-day war with Russia.
The report also criticised Russia, saying its response to the Georgian military strike went 'beyond reasonable limits'.
However, its findings were particularly critical of Georgia's conduct under President Mikhail Saakashvili.
The findings are likely to further damage Mr Saakashvili's political standing in Georgia, and underline Western concerns that have set back his hopes of joining NATO.
The report, which was commissioned by the Council of the EU, said the war was 'the culminating point of a long period of increasing tensions (and) provocations'.
But it added: 'The shelling of Tskhinvali (in South Ossetia) by the Georgian armed forces during the night of 7 to 8 August 2008 marked the beginning of the large-scale armed conflict in Georgia.'
This fell into line with the general view held by many Western governments.
Swiss diplomat Heidi Tagliavini, who led the investigation, said in a statement: 'None of the explanations given by the Georgian authorities in order to provide some form of legal justification for the attack lend it a valid explanation.'
Russia responded with a counter-strike, driving back Georgian forces and pushing further into Georgia.
Moscow later recognised South Ossetia and the other breakaway Georgian region of Abkhazia as independent states, backed by Russian forces.
The report said Georgian, Russian and South Ossetian forces had all violated international humanitarian law in the five-day conflict and the risk of new confrontation 'remains serious'.
It concluded that Russia's first military response to the Georgian assault was legal for defensive purposes, but much of its subsequent military action went beyond reasonable limits.