PETER Howson is hard at work in his Glasgow studio. Classical music plays as he perches on a chair in front of a large canvas, applying paint with deft brushstrokes. The excited barking of his dog Buster announces my arrival, but Howson is so engrossed in the task at hand, that he doesn't look round.

Another few minutes pass until finally, seemingly satisfied with the section he's been focusing on, Howson rises from the chair to greet me. As he moves away from the canvas, I'm able to take in the painting in its entirety.

It is a powerful piece, one that depicts scenes of unimaginable horror, the foreground a tangled mass of bodies and grotesquely contorted limbs. Men and young boys are being corralled by force. Some already lie dead. Others are dying. Or kneeling to await their fate at the hands of blank-faced executioners.

A section of the sky glows red with fire. High up in a tree, unseen by those below, hides a little girl, the sole eyewitness to the brutal slaughter that is unfolding.

Massacre of Srebrenica, which was unveiled at St Mungo Museum of Religious Life and Art in Glasgow earlier this week, serves as a potent reminder of the worst atrocity on European soil since the Second World War.

In July 1995, during the height of the Bosnian war, the genocide of more than 8,000 Muslim men and boys took place at Srebrenica.

Howson is no stranger to the barbarity. He was commissioned by the Imperial War Museum to record the religious and ethnic conflict in the former Yugoslavia in 1992, and a year later appointed official British war artist for Bosnia.

The experience, he says, had a profound and lasting impact. There were moments, admits Howson, when he questioned his sanity. "I have nightmares from Bosnia because the Army took me to places where I saw things I couldn't believe," he says.

Howson observed decaying and mutilated corpses, burnt out buildings and "cleansed" houses, as well as the aftermath of a mortar shelling on two coachloads of refugees. "There was quite a large death toll that day, bodies and brains all over the place," he says.

The 61-year-old Glasgow School of Art alumnus says it has taken almost a quarter of century – during which he has battled drug and alcohol addictions, alongside dealing with periods of severe depression – to come to terms with what he witnessed as a war artist.

Nor was it something he envisaged ever revisiting. When Glasgow Museums initially approached him, it was with the idea of borrowing a painting from his existing portfolio to feature as part of commemorations marking the 25th anniversary of Srebrenica next year.

However, none of the pieces from the 300-strong collection Howson painted during his tenure as a war artist – exhibited at the Imperial War Museum in London in 1994 – were available. He agreed to create a new artwork that will be loaned to Glasgow Museums for an initial three-year period.

When I visit his studio in late October, Howson is still immersed in the creative process. The painting, he explains, has already undergone several incarnations. "I'm not satisfied entirely with it," he muses. "If I did it again, I would make it different.

"There was a lot of red in it before. It has changed quite a lot. There's an infinite amount of possibilities. Tonight, I could change it completely."

He casts a critical eye over the canvas before conceding that, with less than seven weeks until the painting is due to be handed over to Glasgow Museums, any drastic changes at this stage perhaps wouldn't be prudent.

It's a far cry from the Howson who once destroyed a portrait of the Scottish Catholic martyr St John Ogilvie, wiping out nine months' worth of work in an afternoon.

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As we sit down to recount his time in Bosnia, there's a few moments of understandable reticence from Howson. He skirts tentatively around the subject, treading carefully. Later, he will talk more about the deep-rooted effect it had on his mental health.

Howson hesitates as if attempting to dredge the dark recesses of his mind. "It's a long time ago and very bad memories. You don't realise how much at the time it has affected you. Revisiting it now and doing this, it has brought the whole thing back to me but not in such a traumatic way.

"I witnessed a lot of things in Bosnia which were horrific, but nothing like this." He gestures towards the painting. "I had to use my imagination for this."

He was appointed as official war artist in 1993 – one year into the conflict – travelling with the British forces participating in the United Nations Protection Force. Howson first arrived in June that year but fled after less than three weeks, fearing his camp was about to be attacked.

He returned to Bosnia within 24 hours, but only a few days later was hospitalised after contracting dysentery and flown home to Scotland. His early departure didn't go down well in the media.

After six months at home in Glasgow, Howson returned to Bosnia in December 1993. "I went back a second time to try and clear my name because there was such a bad feeling and I was being called all sorts of names for coming back early," he says.

"The second time was a lot easier and I started working even though my art materials were all stolen on the second night there. I didn't have anything to work with apart from some paper. I raked around and found some boot polish and candle wax.

"I used a wax resist technique where I was melting the candle onto the paper. That got me started. It was the winter, so there wasn't as much disease around. I was drinking as well. I suppose that made it a bit easier."

As a coping mechanism? "It was," he says. "But I didn't really need an excuse to drink. I had a problem with drink all through my life. I was able to function. I wasn't staggering about drunk all the time. It was like a constant drip of alcohol."

Howson was based largely in central Bosnia. "Zenica. Travnik. All these places I find hard to pronounce," he says. "The main British base at that point was in Vitez. That is where I was mostly."

The atrocities he witnessed provided the basis for pieces such as Plum Grove (1994) and Bosnian Harvest (1994). "When I was in Bosnia the second time I did about 60 drawings. When I came home I did around 300 pieces altogether.

"But even then, I was struggling when I got back. I was having lots of personal issues and my marriage was collapsing. It was a painful time."

Howson upped sticks to London where he called in a favour from his friend Iain McCall, who had been an assistant war artist. "I persuaded him to lock me in the studio for two weeks without letting me out. It was the only way I could do it. It got me started."

There was widespread controversy over his piece Croatian and Muslim (1994), detailing a rape, which Howson painted from the victim's account. Did he expect such a strong reaction?

"There wouldn't have been any controversy if David Bowie hadn't bought it," he says. "The director of the Imperial War Museum had said that it couldn't be considered authentic because I had done it from victim accounts rather than something that I had witnessed first-hand.

"That caused the controversy. I didn't mean for it to be controversial. But then David Bowie came along and bought it."

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I'm curious as to what reasons the late singer gave for buying the painting. "He just thought it was really powerful," says Howson. "He bought another one at the same time of a dog that had been hung. That was something I had witnessed. It is another powerful piece, but it is not as well known."

Howson flicks through the pages of a catalogue to find the image, an oil on canvas titled Hound (1994), and we gaze at it for a moment.

It seems likely that Howson was suffering from post-traumatic stress disorder upon his return from Bosnia. "It probably was that, even though I hadn't been through anything like some people had; the soldiers had seen a lot more than me. I was very delicate at the time. I was nervous and shaking.

"During the exhibition at the Imperial War Museum, I got a message from my agent Matthew Flowers to say David Bowie wanted to go out for dinner. The prime minister of Bosnia [Haris Silajdzic] had come to London under heavy guard and wanted to meet me too.

"It was such a weird thing. We were all in a private suite at The Dorchester having dinner. Seated round the table was David Bowie, the Bosnian prime minister and the head of Sony Music. Standing behind us were all these armed guards. It was a strange experience.

"David Bowie was quite involved in the politics. At that time, everyone was pro-Bosnia. No one was pro-Serbia or pro-Croatia. On reflection, every single side did atrocities.

"Certainly, though, if you are talking about this massacre [in Srebrenica], that was a definite war crime by the Serbs. That was the biggest massacre since the Second World War. It meant a lot to me at the time. I was fascinated by the whole thing for years after. I still am, the horror of it happening.

"I know quite a lot from being there and talking to the soldiers. It didn't have to happen that way. It was partly the inaction of the United Nations who should have stopped it from happening."

While Howson wasn't in Srebrenica, close to Bosnia's easternmost border with Serbia, he has no difficulty in conjuring an image in his mind's eye.

"I don't think anyone witnessed it because they did it in such secrecy," he says. "That wouldn't be possible to do these days because someone would have had a phone, filmed it and that would be all over the place. But in the 1990s, when this happened, there is no footage of it at all.

"All I have seen is one tiny film someone took of the start of the massacre. It's what it doesn't show that is more horrific in a way. It doesn't show anyone being killed. You see it before they start killing people. That is the only piece of film I looked at. I think it is the only piece of film in existence."

The Bosnian Serb armed forces at the time were under the command of Ratko Mladic, who was sentenced to life imprisonment in 2017 for his role at Srebrenica. Howson looks down at his hands as he speaks.

"Bosnia – the former Yugoslavia – was a holiday destination and it suddenly turned into a blood bath. Neighbours were killing each other. One house would be completely untouched and the one next to it burnt out and daubed with paint.

"Anything that was alive they would kill. I visited a village called Ahmici where there had been a massacre. They killed all the cattle, sheep and pets. They had destroyed the mosque with explosives.

"The entire place had this horrible feeling of death as you walked through. It was frightening. Everywhere was very still and quiet. There were no birds singing. This was an ordinary village and suddenly it was empty. I don't know what happened to the people."

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Howson recalls travelling through rugged mountain terrain towards a beautiful lake. But all was not as it seemed. "The British soldiers told me: 'We know there are lots of bodies in that lake, but we can't get to them because there is too much fighting going on.'"

The memories are coming thick and fast now. On another occasion, he remembers a sudden commotion outside the camp in Vitez. "This Muslim family turned up, about seven or eight people," he says. "They had been ethnically cleansed and were begging to be allowed into the camp.

"But the British Army wouldn't let them in. They said: 'There will be four tanks going by at 4pm tomorrow. That's your only chance. You will have to run behind them to get into the safe area.' Which is what the family did."

Howson has never returned to Bosnia. Would he consider visiting now? "No, because I made a promise to my daughter Lucie that I wouldn't go back. I would always stick to that. I went to Kosovo in 1999 and I only just made it to the border.

"Kosovo was, in some ways, worse. Again, the memories are hazy, but we took over 20 lorries of aid for the refugees in Kosovo.

"The head of the convoy didn't understand the way things were done with the Albanian mafia wanting paid off all the time, otherwise you can't get the aid through. He refused to give them any bribes. It is the only time in my life I have had a gun put to my head."

After the trauma of Bosnia, it seemed an odd choice to go to Kosovo. What was the thinking behind his decision? "It is difficult to talk about, but I went to Kosovo because, bluntly and honestly, I didn't want to go on. I was actually hoping something would happen in a way.

"I don't like saying that too much because my daughter gets upset but it was like a death wish. I have got over that now. But it was a mental collapse, I suppose, because a year after that I was in a clinic for quite a long time."

Howson was treated for alcohol and cocaine addiction at Castle Craig, Peebles, in 2000. He went through several dark periods and spent much of 2012 in the psychiatric unit of a Glasgow hospital where doctors suggested his turmoil stemmed from what he witnessed in Bosnia.

To an extent, Howson agreed. "But I don't think that is the full picture at all," he says. "It is something in me that is very destructive."

Spending time with Howson is fascinating. The inner workings of his unconventional creative process and darkly comic, obsessive psyche were laid bare in the documentary, Prophecy, recently shown on BBC Scotland.

"I think everyone with a mind would want to make sense of what is going on," he states in the film's introduction. "I don't think people realise how thin the veneer of civilisation is. Underneath that veneer is complete anarchy and chaos."

It is a statement that could sum up his time as a war artist in Bosnia. Working on Massacre of Srebrenica in recent months was, in some ways, cathartic. Yet, equally, it served as testament to the heinous acts that scar humanity.

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"It is coming up for 25 years since Srebrenica," says Howson. "When I was 19 or 20, my dad used to tell me stories about the Second World War and it seemed like a long, long time ago. But to him it was still very close and real. With this painting I want to remind people what happened in Srebrenica. It still could happen today. If things go out of control, this is what happens."

Massacre of Srebrenica by Peter Howson, St Mungo Museum of Religious Life and Art, Glasgow. Prophecy is available on BBC iPlayer