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(TMZ) - 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 TMZ ... this particular clip is their best evidence Bieber was under the influence -- because he misses a couple steps. Watch the video and judge for yourselves.

As TMZ 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.