In its petition on Tuesday to the court in The Hague, Legal Action Against War accused the prime minister of intentionally launching an attack on Iraq knowing it would cause incidental loss of life or injury to civilians.
Besides Blair, the group's petition names Foreign Secretary Jack Straw, Defence Secretary Geoff Hoon and Attorney General Lord Peter Goldsmith who advised Blair's government on the legality of the Iraq war.
"The consensus of international legal opinion suggests that the basis for the war was illegal," said Michael Mansfield, a high-profile lawyer with a reputation for taking on tough-to-win cases.
Cluster bombs illegal
Mansfield said the war - which began when US and British forces invaded Saddam Hussein's Iraq on 20 March last year - would only have been legal with UN backing.
Even then, he added, the US-led occupation could not have used more force than was necessary - meaning that it was illegal for warplanes to drop cluster bombs on Iraqi targets.
Legal Action Against War filed its petition as Blair resisted pressure to publish Goldsmith's confidential findings on the legality of the Iraq war, which were presented to the government just days before it began.
Michael Mansfield (R) says
case needs investigation
Clare Short, who quit Blair's cabinet in May last year in protest over the war, claimed that Goldsmith, the government's top legal adviser, was "leant on" to conclude the war was legal.
Speaking at a press conference in London, Mansfield said he was not prejudging the allegations, but felt it was vital for the International Criminal Court to investigate them.
"We are saying there are matters here that prima facie need to be investigated," he explained.
Test of credibility
He styled the case as a test of the credibility of the International Criminal Court, which became a legal reality in July 2003 with a permanent mandate to try war crimes, crimes against humanity and genocide.
"Is the ICC to turn its back on and say it will only look at banana republics or African states or countries that have greater vulnerability than the leaders of the West which can operate above and beyond the rule of law?"
Lawyer, Legal Action Against War
"Is the ICC to turn its back on and say it will only look at banana republics or African states or countries that have greater vulnerability than the leaders of the West which can operate above and beyond the rule of law?" he asked.
Mansfield acknowledged there would be "extraordinary reluctance" in The Hague to indict serving government ministers, but there was nevertheless "a real case" they should be investigated.
Support for the petition was voiced by Lord Stoddart, who was formally expelled from Blair's governing Labour Party in January 2002 after increasingly falling out over its policies.