Phil (Steve McFadden) has infuriated Jack in recent months by trying to cause problems for his relationship with Sharon Rickman, and the love triangle intensifies in upcoming episodes as Jack angrily attacks his rival.
As revealed in this fortnight's All About Soap magazine, Jack (Scott Maslen) will fear he has killed Phil as the businessman is left lying lifeless at The Arches following their altercation.
The latest story development begins when a jealous Jack encourages Sharon (Letitia Dean) to stop running the R&R club for Phil, pointing out that she could have a better-paid job elsewhere.
When Sharon considers this suggestion, Phil is quick to respond by offering her a stake in R&R. However, Jack is at the end of his tether over Phil's close bond with Sharon, and heads over to The Arches to warn him off.
A defiant Phil refuses to call off his deal with Sharon, so Jack punches him and Phil falls backwards into the pit at the garage.
With blood trickling from an unconscious Phil's ear, Jack flees the scene in horror as he wonders what to do next - and he even considers running away from Walford to escape the repercussions.
Jack eventually confides in his brother Max (Jake Wood) over what has happened, but is too frightened to check on Phil to see if he is still alive.
As Jack drowns his sorrows in the Queen Vic, he later faces a mixture of relief and worry when Phil staggers into the pub in a state.
Although Phil initially has no memory of what happened to him, he soon remembers enough to announce Jack's guilt to the whole pub. As Phil is rushed to hospital, viewers will be left to wonder what the consequences will be for violent Jack.
EastEnders airs these scenes on Monday, February 25 and Tuesday, February 26 on BBC One.
Digital Spy Soap Scoop video - watch Amie Parker-Williams reveal the biggest storylines coming up in EastEnders, Coronation Street, Emmerdale and Hollyoaks next week: