Horowitz, who has written a new James Bond book, made his comments about Elba - best known for his roles in "Luther" and "The Wire" - in an interview with a leading publication, reported BBC online.
Horowitz apologised via Twitter for the use of the word "street" to describe Elba's acting.
"In the article, I express the opinion that to my mind Adrian Lester would be a better choice but I'm a writer not a casting director so what do I know? Clumsily, I chose the word 'street' as Elba's gritty portrayal of DCI John Luther was in my mind but I admit it was a poor choice of word. I am mortified to have caused offence."
"I'm really sorry my comments about Idris Elba have caused offence...," he added.
Elba has long been touted as a possible successor to Daniel Craig as the fictional secret service agent.