Mr Fitzsimons could face the death penalty if convicted of the murder of the men, both 37, in Iraq.

He is also accused of wounding an Iraqi and is continuing to be questioned by Iraqi authorities.

He was in the army for eight years and had been unemployed before being taken on by ArmorGroup.

Mr Fitzsimons’ family have been calling for him to be tried in the UK.

Mr Tipple, who has just returned from Iraq after talks with officials, told the BBC his legal team would be putting “a lot of pressure” on the UK government.

He added: “His duty of care was ignored and when he left the Army he was suffering symptoms of post-traumatic stress disorder.

“We intend to call public meetings, the family campaign is growing and the government will feel the pressure of that campaign – rest assured of that.

“Daniel Fitzsimons faces the death sentence. This is too serious a matter just to leave as it is. We have to apply that pressure and we intend to.”

Mr Tipple said his client was as well as could be expected “under the circumstances” and described his cell as “effectively a dungeon”.

Michael Fitzsimons, his brother, said: “Given the circumstances with Danny’s mental health he should never have been in Iraq.”

A spokesman for the Foreign and Commonwealth Office (FCO) said the investigation was a matter for the Iraqi authorities.

But he added that “should it become clear there are concerns about the ongoing legal proceedings we will intervene”.

The Green Zone is a heavily protected region of Iraq’s capital city, which houses Iraqi government, coalition headquarters and most embassies.

In January Iraqi forces took over responsibility for the security of the zone.