Top lawyers from Greece are travelling to the International Criminal Court in The Hague on Monday to file a lawsuit against senior UK officials.
They will accuse the prime minister and other senior members of the government and military of breaching international law by attacking Iraq.
The Athens Bar Association (ABA) believes it has strong evidence and is seeking the indictment of Mr Blair.
The ABA will file the lawsuit with the prosecutor of the ICC who will then have to decide whether it merits further investigation.
The court was established to try cases of war crimes, genocide and crimes against humanity.
Among those named in the lawsuit are Mr Blair, Defence Secretary Geoff Hoon and Foreign Secretary Jack Straw.
The Greek lawyers announced in May they would try to take legal action against the British Government and military.
They said the war in Iraq breached international treaties such as the Charter of the United Nations, the Geneva Conventions and the ICC's own Statute.
But the British Government has always argued that the invasion of Iraq was in accordance with international law.
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