EXCLUSIVE

Fifth Harmony had only 4 members who were free last night, because Lauren Jauregui was busted for weed Tuesday night while trying to get on a plane at Dulles Airport. 

Law enforcement sources tell TMZ ... Lauren was stopped at a TSA checkpoint and forced to submit to a secondary search, when a TSA staffer found what appeared to be a bag of marijuana in her carry-on luggage. 

Cops were flagged down in the terminal and they promptly arrested the singer. 

It appears Lauren was heading for Brazil where the group is set to perform Wednesday night.