Justin Bieber -- Stumbles in DUI Jail Video ... Cops Think It's Smoking Gun

Justin Bieber Stumbles in DUI Jail Video Cops Think It's Smoking Gun

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Here it is ... the footage of Justin Bieber in jail last month following his DUI arrest ... the same footage Bieber tried to keep out of the public eye ... but it may actually HELP him.

The video shows Bieber taking a sobriety test while inside the jail facility -- trying to walk a straight line.

Law enforcement sources tell us ... this particular clip is their best evidence Bieber was under the influence -- because he misses a couple steps -- but we gotta be honest ... it's not THAT bad. Watch the video and judge for yourselves.

As we reported, Justin's lawyers tried to block the release of the footage -- after video surfaced of him getting patted down in jail -- but the judge told them to pound sand.