A SAS sergeant at the centre of a Ministry of Defence investigation into alleged "mercy killings" has claimed that the execution of enemy casualties is a standard practice in Britain's Special Forces.

It emerged last week that Colin Maclachlan, 42, is under investigation after he claimed he was part of an SAS team that killed two or three mortally wounded Iraqi soldiers in 2003.

Sergeant Maclachlan, from Edinburgh, has now claimed that mercy killings on the battlefield are routine.

He told the Mail on Sunday: "Of course they are. You would do the same.

"If I put you in that position and I gave you a gun, I put you through a battle so that you were in that mindset - mud, blood, and you see someone like that, you know you can't save them and they're pleading you for it, you'd pull the trigger yourself."

However, the former soldier acknowledged that such mercy killings were a legally murky area.

He added: "There's no law that says you can finish him off. None. You have not legal stature. It's murder. But we're human beings, we have compassion. We know how we would want to be treated in that place."

Another anonymous former soldier quoted by MoS said that mercy killings "are part of the job and role and are an unwritten rule". However, he added that Sergeant Maclachlan had "opened up Pandora's Box" by speaking about the issue publicly.

A spokesman for the MoD said: "Our Armed Forces are held to the very highest standards and any credible allegations of criminal behaviour will always be investigated."

It comes after Sergeant Maclachlan was last week accused of lying about being taken hostage and tortured while working on an undercover SAS mission in Basra in 2005.

Fellow SAS men accused the former soldier of inventing the story, while the MoD also undermined the claims by confirming Sergeant Maclachlan had left the Army four months before the 2005 incident.