British Foreign Secretary Jack Straw said in a public statement, "To kidnap and kill anyone is inexcusable but it's repugnant to commit such a crime against a woman who has spent most of her life working for the good of the people of Iraq."
Tony Blair sent a message of sympathy to the family of Hassan, who live in Ireland. He said he "shared their abhorrence" at the murder, the London Evening Standard reported.
Irish Prime Minister Bertie Ahern said, "Those responsible for abducting Margaret Hassan stand condemned by everyone throughout the entire international community."
Arabic television channel al-Jazeera said Tuesday that it had received a video showing a militant firing a pistol into the head of a blindfolded woman in an orange jumpsuit.
Experts have checked the video and concluded that it is genuine.
Her Iraqi husband begged her captors Wednesday to return Hassan's body to him.
Tahseen Hassan said in a television plea, "I need her. I need her back to rest in peace."
Churches in Ireland are holding services for the Irish-born Iraqi-British national, who had lived for thirty years in Iraq and had converted to Islam.