According to the Daily Star, Porter left Miller after accusing him of approaching girls in nightclubs for sex. Although Miller apparently denied any wrongdoing, Porter is believed to have ended the relationship.
A supposed friend of Miller claimed: "He's acted like a complete idiot. Kirsty is a lovely girl and gave him chance after chance.
"He was going out acting like a single lad. Kirsty is a strong girl and decided she couldn't go on with their relationship."
The insider added: "She really loved Danny and still can't believe the way he treated her."
Porter originally split from Miller in March last year for a reportedly similar reason, before the pair reunited in the summer.
Porter herself appeared on Emmerdale for a year from 2010, leaving her role of Roz Fielding in April 2011.
Meanwhile, Miller quit his high-profile part on Emmerdale late last year, filming his final scenes in March.
> Emmerdale exit hasn't sunk in, says Danny Miller