'Harry Potter' Star Robbie Coltrane -- Hagrid Hospitalized After Flight

"Harry Potter" star Robbie Coltrane got off a plane Thursday and was immediately hospitalized after suffering severe flu-like symptoms during the flight ... TMZ has learned.

A rep for Coltrane -- who played Rubeus Hagrid -- tells us the actor fell ill during a British Airways flight from London to Orlando.

We're told paramedics met Coltrane at the gate, and took him to the hospital ... where he'll remain overnight for treatment.

Coltrane went to Orlando for a Harry Potter convention ... and we're told Michael Gambon -- who played Dumbledore -- was on the same flight.

Someone on Twitter claims Coltrane was drinking heavily in a London airport lounge before the flight. If it's true, we don't know yet if that contributed to his illness.