Shaun Stewart, 22, attacked his victims on the venue’s dance floor – leaving one of them unconsious for four-and-a-half minutes.

Prosecutor Malcolm Macleod told Dumbarton Sheriff Court that Stewart was a skilled amateur pugilist with aspirations to compete at the Commonwealth Games, a claim disputed by his lawyer, who said his client’s ability was not at that level and he only attended the gym to ‘keep fit’.

But a video clip of one of Stewart’s amateur bouts has been posted on the website YouTube, and shows him celebrating a knock-out victory. During the one-and-a-half minute clip, Stewart can be seen walking away after twice leaving his opponent prone on the canvas in a scene not too dissimilar to the bar assault described in court.

Fiscal depute Mr MacLeod told the court the attack in Helensburgh’s Clyde Bar involved Stewart delivering a total of 14 punches between both victims.

Stewart, of East Princes Street, and the off-duty servicemen were enjoying an evening out at the same venue on July 20 last year when, at around 11.30pm, a heated argument started involving Stewart and one of the victims.

The situation escalated when one of the men gave Stewart what was described as a ‘playful’ slap on the cheek which attracted two less playful punches from Stewart, which the prosecutor said were delivered in ‘rapid succession’.

Mr Macleod told the court Stewart continued to punch the man a number of times causing him to fall to the floor. He then turned his attentions on the second man who also found himself on the deck.

But when they got back to their feet Stewart attacked again and this time one of his victims was knocked out.

Mr Macleod said: “The accused came from behind and punched him on the head, knocking him on to the floor and rendering him unconscious. The complainer remained prone and unconscious on the floor for about four and a half minutes whilst being attended by door stewards. The accused made off and the police were contacted.” Officers attended the bar and later caught up with Stewart at around 2am at the junction of John Street and West Clyde Street. One of the victims was treated at the Vale of Leven hospital while the second man who had been knocked out refused medical treatment.

At a subsequent court hearing Stewart, formerly of Clydebank Osprey Amateur Boxing Club, admitted assaulting both men by repeatedly punching them on the head to their severe injury at the Clyde Bar, West Clyde Street.

Stewart’s defence agent Mr Paul described his client as ‘polite and attentive’ and said he was anxious to avoid a custodial sentence. On the night in question he lashed out because he ‘felt as though he had been backed into a corner’.

He said that although Stewart had attended a boxing club it was more to do with fitness and he was not at the level where he would enter the Commonwealth Games or anything like that.

Mr Paul acknowledged that his client had a previous conviction of a similar nature but, arguing in favour of a non-custodial option, he said that Stewart had overcome a troubled background and was now in steady employment and in a stable relationship.

Sheriff Carol Kelly told Stewart she had to take into account the fact that he had a similar previous conviction from three years before, adding: “Someone who has trained as a boxer should be aware of the danger of a potential attack such as this.” She sentenced Stewart to nine months in prison and imposed a supervised release order for a period of four months.