Ian McKellen Hospitalized After Falling Off Stage During London Performance

4:30 PM PT --  A rep for "Player Kings" tells TMZ Ian McKellen is doing well after his fall, saying ... "Following a scan, the brilliant NHS team have assured us that he will make a speedy and full recovery and Ian is in good spirits."

"The production has made the decision to cancel the performance on Tuesday 18 June so Ian can rest. Those affected will be contacted by their point of purchase as soon as possible tomorrow."

"Thank you to doctors Rachel and Lee who were on hand in the audience and to all the venue staff for their support.”

Ian McKellen stepped into the spotlight in London and ended up in a hospital ... this after taking a nasty fall during a theater performance this week.

The actor -- who turned 85 last month -- was hospitalized Monday after losing his footing in a "Player Kings" fight scene and falling off stage ... this according to multiple reports out of the UK.

Ian plays John Falstaff in 'PK' -- a new West End production of Shakespeare's Henry IV parts one and two -- and he was circling a battle scene when he tumbled off the front of the stage, screaming in pain as he hit the deck.

The full extent of Ian's injuries is unclear ... but it was serious enough for theater staff to run to his aid. The play was then scrapped entirely and the audience was evacuated from the theater as Ian was taken for medical attention.

Ian's been performing in "Player Kings" at the Noël Coward Theatre in London since April, and the production has only a few weeks of shows remaining.

He's been outspoken about turning down lots of roles in his old age, but was open to doing this play, and recently said he'd consider playing Gandalf again in a potential 'Lord of the Rings' movie ... although, he admits whenever he gets a script now, he considers the possibility it may be his last.

Hopefully, Ian's doing alright ... we've reached out to his team, no word back yet.