Omid Scobie -- who wrote a controversial book on the royal family last year -- shared a tweet of him setting an alarm for 6 PM on Friday ... the time in England when Kate Middleton shocked the world by revealing she has cancer.

And, right after the Princess of Wales revealed the info, two things happened -- Scobie took the tweet down, and social media raked him over the coals.

Comments calling the tweet tasteless and attacking Omid's character ran rampant across X ... and the journalist ultimately had to address the online controversy after several media outlets picked up the news.

In another tweet posted Friday, Scobie explains he didn't know what the announcement was about, and only wanted to alert his followers to the news -- deleting his post shortly after watching Middleton's video.

Scobie expresses his frustration with the outlets who reported the story, saying they ran with the post as if it were a sick "countdown" to the news ... which he says was never his intention.

It's not totally shocking some people may think Omid was making light of the news ... 'cause he's quite the controversial figure in the royal realm. Remember, Scobie wrote the book "Endgame" which made some serious claims about the royal family.

Among them, two senior royals were alleged to have made racist comments about Meghan and Harry's children. Though they were supposed to go unnamed, a Dutch translation of the book listed them ... as King Charles and Princess Kate.

Scobie's said the two were erroneously identified ... but, the book left a sour taste in many people's mouths -- and it appears Scobie's no longer getting the benefit of the doubt, especially when it comes to the Princess of Wales.