The star-crossed lovers were supposed to take the stage for the first time in front of paying audiences Saturday night at London's Duke of York theater ... but, Deadline reports ticketholders received an email canceling the show Friday.

According to the outlet ... showrunners said technical aspects of the play needed further refinement before audiences could see it -- so, sounds like they couldn't quite iron everything out in rehearsals.

"Romeo & Juliet" is supposed to have more previews early next week ... no word yet on whether those will go ahead as planned.

Holland's show has already had to deal with a series of issues leading up to the previews ... remember, his costar Francesca Amewudah-Rivers has faced a series of racist attacks over the last few moments for playing the titular heroine.

The Jamie Lloyd Company said in a statement the attacks must stop and reiterated its commitment to making sure the production itself ran smoothly despite the vicious attacks ... unfortunately, doesn't seem like that's possible at the moment.

The official opening night for 'R&J' is set for May 23 ... though with all the delays, it'll be interesting to see if that's still in the cards.