A.J. McLean from the Backstreet Boys has gone into rehab again … TMZ has learned.

One source tells TMZ McLean has been “out of control” in the last few months because of substance abuse, and it was interfering with plans for the Backstreet Boys’ upcoming tour with New Kids on the Block.

Sources say on a Backstreet Boys cruise last month … A.J. appeared under the influence much of the time.

McLean has battled addiction for years. He was in rehab in both 2001 and 2002 for alcohol and cocaine abuse.

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