The former British army marksman who holds the record for the longest range sniper kill in combat says he has lost all trust in the armed forces because of the way he has been treated.

Craig Harrison, who served as a sergeant in the Blues and Royals cavalry regiment, killed a Taliban machine gunner with a shot over a distance of 2.5 kilometres - the length of 25 football fields, and almost twice as long as his rifle's listed effective range

The seemingly impossible shot saved his comrades and saw him commended for extraordinary gallantry.

"I just believed that I had to do something to get 12 men that were stuck in vehicles out of the situation," he recalls.

But despite serving in Bosnia and making two tours each to Iraq and Afghanistan, including commanding the team where he took his record-breaking shot in 2009, the former British sniper says his regiment snubbed him when he developed post-traumatic stress disorder (PTSD).

"They didn't even say thank you. I joined when I was 16 and since all of this has happened I felt abandoned, absolutely abandoned by my regiment," he said.

"Nothing made me prouder than that regiment. I spent 22 years loyal to that regiment, putting my life on the line doing tours, and they just hung me out to dry.

"My trust in people, the armed forces — it's gone."

Mr Harrison says he has been called a "coward" by some people on social media because people misunderstand what it takes to be a sniper and think it is a clinical, clean, distant way of being in battle.

In fact, he says it is surprisingly "intimate".

"You're looking through a high-powered scope, you're observing that target, you know how he talks, you get to know his routine through the day, and then you have to take him out," he said.

"So he almost becomes somebody you know because you've observed him for that long period of time and then you have to kill him. It's quite personal."

Mr Harrison writes about his experiences in a new book, The Longest Kill.

But he says he was left feeling hollow after turning himself into a "killing machine".

"It's a feeling you're empty inside, but when you come home then it's what you've got to think about," he said.

"And I'm lucky I've got a strong wife. She talks to me and I talk to her about things, but I don't talk about everything to her because it's not fair on her to burden her with killing people."

Digging up mass graves triggered PTSD

Mr Harrison says he still remembers all the people he had to kill.

"I can close my eyes now and I have to take medication to help myself sleep because I can smell them, I can see them - every person I've taken their lives, I can see them in my head," he said.

"I can see them spitting and smoking, walking, and I can see me taking them out.

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"People you don't want to shoot because you have to, because they're supplying the Taliban with weapons and you need take targets out."

He says he did not realise he had severe PTSD until his wife made him see a doctor because of the way he was behaving after his 2009 tour of Afghanistan.

"When I go into a room I check every door. Nightmares, horrible, horrible, vile nightmares. Feeling you're getting suffocated in clouds," he said.

"You think if there are people walking behind, you think everyone's out to get you. You just want to shut yourself away in a dark room."

He believes it was in Bosnia when he first began to experience PTSD.

"I think it was digging mass graves up. I remember there was an incident where the Serbians went to this wood line and all the Albanians were in this wood line and they just hosed this wood line down," he said.

"Basically we went up there after it happened and there were suitcases in the trees, clothes in the trees, and we had to put an arm to a body and match it, stuff like that.

"I don't think the army knew much about PTSD as much as they should have."

Mr Harrison says that after his tour of Bosnia the army's trauma counselling consisted of a two-sentence conversation.

"I can remember it clear as day. You go into an office, there'd be somebody there from welfare and they say, 'How're you feeling? Are you doing OK?'," he said.

"You're not going to say, 'Yeah, I feel pretty down in the dumps because I done this'. You just want to go home.

"You say, 'Yeah, I'm feeling fine'. 'OK you go'. And that's how it used to be."

Iraq, Afghanistan wars 'like table tennis'

The British army had improved its PTSD counselling by the time he had returned from his final tour of duty, but that did not mean his regiment was prepared to accept a war hero with a mental illness.

He was recommended for a Military Cross for bravery but his senior officer declined on his behalf.

Mr Harrison says he felt betrayed by that.

"I should be the one that declined it, not another person, because I couldn't cope with stress," he said.

The army officers also made mistakes in their handling of the media over his record-breaking sniper shot.

The consequences were devastating.

His identity was revealed and he became a target for Al Qaeda and was threatened with beheading; his family went into hiding for three years.

Mr Harrison now believes his actions were worth all his pain because he saved the lives of the men he was commanding, but that the personal cost was not worth it.

And he says he does not support the latest commitments to Iraq.

When he asked a commanding officer on his second tour of duty in Iraq what he believed the troops were doing there, he says he never got a straight answer.

"In my opinion, it's like a game of table tennis. You know, we went to Iraq, then we went to Afghan," he said.

"We left Afghan, now we've gone back to Iraq because of ISIS and they've gone into other places.

"And I guarantee you, once all that's calmed down, we'll go back into Afghan again.

"And if they send British troops in after ISIS, that'll be the biggest mistake we've ever made."