The father of a serviceman killed in the conflict alleged that the former prime minister used WMD as a "way in" to the war despite being told by former UN weapons inspector Hans Blix that the weapons did not exist.
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Richard Green urged members of the Iraq Inquiry to look closely at how arguments about WMD were used in the run-up to the March 2003 invasion.
His son, Royal Navy Lieutenant Philip Green, 30, from Caythorpe, Lincolnshire, was one of seven personnel killed when two Sea King helicopters collided over the northern Arabian Gulf two days after the start of the war.
Speaking at a special session of the inquiry in central London for bereaved families, Mr Green said: "It is the weapons of mass destruction that this centres upon.
"I believe that Blair used this as a way in. He couldn't do it with regime change because that would not have been allowed."
Mr Green referred to an interview in which Mr Blix said he and his team had visited Iraq regularly and found no evidence of WMD.
He said: "They reported this back to Downing Street and to the UN, and it would appear from the aftermath that this was ignored."
Mr Green was greeted by calls of "hear, hear" from other families as he dismissed the claim that Iraq could launch WMDs at British interests as "a load of rubbish".
The session also heard from Reg Keys, whose son, Lance Corporal Tom Keys, 20, from Llanuwchllyn, near Bala in North Wales, was one of six members of the Royal Military Police killed by a mob in southern Iraq in June 2003.
Mr Keys said the inquiry should look at the legality of the war, concerns that British troops were not properly equipped and whether there had been justice for those killed.
He said: "We were led to believe this was a just cause.
"I think it's becoming abundantly clear now from the evidence already given that it was not a just cause."
This was the sixth hearing the inquiry has held for families of those killed in the conflict.
Previous sessions have taken place in London, Edinburgh, Manchester, Bristol and Belfast.