A Missouri police officer's full account of why he shot dead unarmed teen Michael Brown has been revealed for the first time - and he describes a frantic struggle inside his police car during which Brown grabbed his gun, twisted it around and pointed it at him - making him fear for his life.

Darren Wilson's testimony runs contrary to most eye-witness accounts and significantly makes the claim that Brown, 18, charged him after the initial struggle and never raised his arms in surrender.

Wilson, 28, paints a picture of an officer in distress during the afternoon of August 8 as a belligerent Brown, who was 6-foot-4 and nearly 300lb, tried to steal his sidearm and turn the weapon against him.

Further compounding Wilson's version of events was the revelation that Brown had marijuana in his system when he was shot. The police also say Brown had marijuana on him at the time of his fatal run-in with Officer Wilson.

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Confrontation: New details of an account by Darren Wilson (left), the police officer who killed Michael Brown (right), describes a violent encounter in which Wilson was allegedly hit in the face several times before firing a gun which Brown was trying to wrest away from him

Aftermath: Pictured above in a grainy cellphone photograph grab, Wilson stands over Brown after the fatal - and hotly-disputed incident

Darren Wilson’s testimony, as leaked to the St Louis Post-Dispatch, is the first time a full account of the shooting in Ferguson, Missouri, on 9 August has been revealed – and is sure to further stoke the controversy surrounding the shooting.

It comes after partial elements of Wilson' version of events were leaked last week and the same day that the former head of the St. Louis County Police said he believed Wilson would most likely not be indicted by a Missouri state grand jury.

The police officer’s dramatic account of events depicts a life-or-death struggle inside his squad car with the 'incredibly strong' teen – after Brown allegedly punched him in the face and reached for his gun.

And, significantly, Wilson claims he shot Brown dead as the teen charged at him on the street – after the alleged attacker apparently retreated after he was shot in the hand during the struggle in the car.

A grand jury decision on whether Wilson should be indicted on criminal charges is expected within weeks.

A separate leaked autopsy report also appears to show Brown had marijuana in his system when he was shot.

The account, based on statements Wilson made after the encounter, was conveyed to the Post-Dispatch by a source familiar with the documents.

Other witnesses have dismissed Wilson's version of events, and the Brown family's lawyer said 'the story doesn't even make good nonsense'.

In the account, Wilson told how he was driving down Canfield Drive after dealing with a call about a sick baby when he spotted Brown and his friend Dorian Johnson walking down the middle of the road.

He said he told them to get onto the sidewalk, and was met with abuse from the pair.

The statement said Wilson then noticed that they were carrying cigarillos, and that Johnson looked like a man described in recent radio alerts as a suspect in a convenience store theft in which 'blunts' were taken.

The police officer then said he parked his Chevrolet SUV, and called for assistance. But, he claims, as he tried to get out he was met with a punch in the face from Brown, who allegedly struck through the car's open window.

Immediate response: In this image investigators crowd round Brown's body immediately after the shooting

Wilson said he had no choice but to draw his pistol when faced with the 'incredibly strong' teen. Brown was 6-foot-4 and nearly 300lb at the time of his death.

Wilson said he couldn't use his pepper spray in the close quarters because it would have hit him too. His baton was out of reach at the back of his belt.

In the statement Wilson told how Brown grabbed at the pistol and continued to hit him in the face.

The officer, who said he almost passed out in the attack, then told how he felt Brown, whose strength he found remarkable, twisting the barrel towards his own body.

At one point, the muzzle of the gun was pointed at Wilson's own hip.

Wilson went on to describe how he jerked back inside the car, loosening Brown's grip on the gun enough for him to squeeze the trigger.

Vigils: Demonstrators - pictured on October 20 - have continually held vigils for Brown since the shooting in August. They are pictured above on Canfield Street, where he died

Protests: Thousands have taken to the streets of Ferguson in outrage over the incident

But, he said, the first time nothing happened - because Brown had his finger in the gun's hammer, which stopped it from firing.

Separate accounts of the events which have been made public have also claimed there was a scuffle of some sort inside the police car - citing blood and skin samples picked up in investigations.

When Wilson pulled the trigger again, the pistol went off, hitting Brown's hand and sending glass cascading around the car.

Evidence presented by the Post-Dispatch from the autopsy report also seems to support a close-range impact. The document told how the entry wound in Brown's hand had no 'stippling' - a feature of mid-range impacts - suggesting a point-blank release.

In his account, Wilson then described firing a second shot - which missed. Brown then ran away.

Response: Police have donned riot gear to meet the protesters in ugly confrontations

Wilson said he was left unable to radio for backup because his transponder had been knocked away from the right setting in the chaos.

So, on his own, Wilson said he got out of the car and gave chase. It was at that point when Brown turned.

Wilson said Brown then 'charged' at him and wouldn't stop when he yelled. He started firing, and hit four times. One shot, Wilson said, hit Brown's forehead and brought him down.

He also denied - as many witnesses have claimed - that Brown had his hands up in a gesture of surrender.

Wilson's extensive account is at odds with several others which have emerged in the wake of the killing, which has sparked widespread and ongoing protests in the Missouri town.