The killer of Irish-born aid worker, Margaret Hassan, has “gone missing” from prison after he was convicted of the 2004 killing of the worker.

Deirdre Manchanda, the sister of the director of CARE International in Iraq, said the family's lawyer has been told by authorities that Ms Hassan's killer could not be found at the prison where he was being held while awaiting a retrial.

Ali Lutfi Jassar was sentenced to life in prison last year by Baghdad’s Central Criminal Court but was granted a retrial after he argued that his confession had been made under coercion.

The architect claimed that he was outside of Iraq at the time of the killing: Iraqi authorities are said to be frantically looking for evidence to prove that this was not the case.

He was due to appear for retrial on Thursday, before he “went missing” from prison.

A statement from Manchanda, the family’s lawyer, read: “We have just received the very disturbing news that Jassar cannot be located in the prison facility in which he was being held. “The Assistant Director General of the prison has confirmed that he cannot locate him. “Our lawyer in Baghdad expressed his concerns sometime ago about ALJ’s non-appearance at any of the Appeal Hearings.”

Dublin-born Mrs Hassan, 59, was the director of humanitarian group Care International in Iraq, was taken hostage on her way to work in Baghdad in October 2004 and shot dead just under a month later.

The Dublin-born Catholic, who had joint British, Iraqi and Irish nationality, was married to an Iraqi and had lived in Iraq for 30 years.

A British Foreign Office spokeswoman said: “We stand by to provide consular assistance for Margaret Hassan’s family, and want justice to be done for this dreadful crime committed against someone who dedicated her life to helping all Iraqis.”

A Foreign Office spokesman said officials were "concerned". "We are working closely with the Hassan family lawyer and the government of Iraq in order to get clarity on where he is detained," he said.