Actor Adam Deacon will be sentenced for posting abusive messages on social media about his former mentor and Adulthood director Noel Clarke.
Deacon, 32, is expected to receive a community sentence after he was found to have sent Mr Clarke a "barrage" of abusive and threatening messages following a disagreement over the title of hit film Anuvahood.
He branded Mr Clarke a "bully" and accused him of sabotaging his career in hundreds of "consistent and continuous" posts on Instagram and Twitter, Hammersmith Magistrates' Court in London heard last month.
Once good friends, the pair stopped talking after the dispute over the 2011 film, in which Deacon made his directorial debut and for which he won a Bafta.
Mr Clarke told the court that he helped launch Deacon's career by selecting him to star in his films Kidulthood and Adulthood.
Finding Deacon guilty, District Judge Shenagh Bayne told him: "I am satisfied that all of the messages that you sent out were sent out in the context of an ongoing grievance that you have got with Noel Clarke. And that clearly is a long-standing grievance - it goes back many, many years."
She said he would have known the response his messages would receive on social media and that, by posting pictures of Mr Clarke's family, he had made "veiled threats".
Adding that these threats "exacerbated" the harassment Mr Clarke felt, she continued: "The sheer number of messages that you sent is indicative of your intention to harass Mr Clarke.
"You were aware of the effect that your behaviour would have.
"You clearly were trying to elicit a response from Mr Clarke otherwise why post #wallofsilence, #industry and #standuptobullies.
"I am satisfied so that I am sure that you intended to cause him harassment and distress because of your grievance that goes back a long time."
Mr Clarke, who starred in Doctor Who, said he was so worried about the content of the messages Deacon posted that he informed his children's school, which subsequently changed security codes around the building.
"You just don't know what is going to happen," he said.
Even after Mr Clarke went to the police in November 2014, Deacon continued to post abuse, calling his fellow star a "snitch".
Deacon is best known for playing the main character, Jay, in the films Kidulthood and Adulthood, which saw him appearing alongside Mr Clarke.
Giving evidence, Deacon claimed he had not harassed Mr Clarke, who seemed to "have it in" for him from the beginning.
"He would bully and belittle me on set in front of people - it would be a constant thing," the actor said.
He told the court he was only paid around £1,500 for his role in Kidulthood, despite being told by Mr Clarke that he would "make a lot more".
However, the actor admitted posting some messages on Twitter making reference to bullying, but insisted these were not directed at Mr Clarke.
"I didn't '@' him, I didn't mention him. That's why I'm so confused as to why there's a harassment charge," he said.
The actor, from Bethnal Green, east London, who is also a rapper and played firearms officer Robbie in Channel 4 police drama Babylon, which aired last year, was released on bail.
Judge Bayne said she would be considering "a sentence within the community - I certainly wouldn't be considering anything greater than that".