Kidulthood actor Adam Deacon has been found guilty of posting abusive messages online about his co-star and writer Noel Clarke.
Deacon, who was also directed by Clarke in the film's sequel Adulthood, sent a "barrage" of messages via social media after the pair fell out.
The dispute was over the title of a third film called Anuvahood, which Deacon co-wrote and directed in 2011 - three years after the release of Clarke's second film.
The film was commercially successful and won 32-year-old Deacon a BAFTA rising star award.
Clarke, who also starred in Doctor Who, gave evidence at the trial at Hammersmith Magistrates Court saying that he had helped launch Deacon's career.
After the initial falling out, he told the court, he began to receive abusive messages - including death threats - from Deacon.
Clarke said: "We fell out in 2011 due to the fact that the films that I had made - Kidulthood and Adulthood - were very popular and in 2010 Mr Deacon asked if he could do a film which was a spoof of the film that I had done."
The court heard that Clarke initially helped Deacon with the project but the arrangement came to an end when Deacon signed a contract that cut Clarke and his team out of the project.
He said that he continued to support him with the venture so long as he did not use the title Anuvahood.
Clarke said Deacon verbally agreed not to use the name, but then reneged on the arrangement.
It was shortly after this, he told the court, that he began to receive the messages which, he said, caused his family "stress".
"They were abusive in nature, some calling me a bully, some saying I was sabotaging him. They went on to include inciting people to hurt me, and sent videos on Instagram."
One of the photos posted was of Clarke's young family accompanied by the message "you have to love family" and a picture of a gun.
Eventually, after a series of messages, Clarke went to the police in 2014.
Finding Deacon guilty, District Judge Shenagh Bayne told him: "You were aware of the effect that your behaviour would have.
"You clearly were trying to illicit 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."
Deacon, from Bethnal Green, east London, is also a rapper and played a police firearms officer in Channel 4's Babylon.
He showed no reaction as he was convicted and was released on bail.
He is due to be sentenced at the same court on 20 August.