Law enforcement sources tell us ... 44-year-old Sean Waltman was flying out to a wrestling event in the United Kingdom when he was stopped at Customs and officials discovered the drugs.

We're told airport police were called to the scene and Waltman was arrested on the spot. He was transported to a nearby jail where he was booked for possession of a controlled substance.

To make things worse ... during the process, officials noticed an outstanding warrant for Waltman stemming from a prior DUI arrest.

Bail was set at $35k. He's since been released.

Waltman has struggled with substance abuse in the past -- he's been arrested for drug possession on multiple occasions. He also told Steve Austin he "got real bad" on crystal meth when he first moved out to L.A.

Waltman was also in a prior relationship with Chyna -- who died from drugs and alcohol after a battle with substance abuse.

We reached out to X-Pac's rep who told us the wrestling superstar is "fine" -- but didn't have any other details.