Lomas was accused of kissing carer Katie Moores in his hotel suite after a night out, and Marsh is said to be worried that more infidelity claims will emerge.
The couple had been arranging an intimate wedding followed by a party and were hoping to secure a six-figure magazine deal.
Marsh, who plays Michelle Connor in the ITV1 soap, has told the former Hollyoaks actor that he has "one last chance to save our relationship", The People reports.
"Kym has chosen to believe Jamie's story that nothing happened because deep down, despite the hurt and anguish, she wants them to stay together. But the wedding is definitely on hold until this blows over," a source said.
"The first Kym knew about it was when she received a call from the ITV press office saying a story was going to be printed. She just thought the worst - that he'd cheated on her. It sounded like she'd made her mind up already - that he was guilty.
"She was screaming and in floods of tears. Jamie explained that absolutely nothing had happened that night. He was drunk and a few people went back to his room for drinks.
"To be honest, he doesn't actually remember much from the night out but is adamant he didn't kiss the girl or do anything. Kym told him he was an idiot for inviting people back to his room, even if there was a group of them and it was innocent.
"She's told him she needs to build up her trust with him again before they start planning the wedding. What happened that night has sown a few seeds of doubt in her mind. It's his last chance. Kym is an independent woman and she won't let him mess her around."
Marsh was recently forced to deny claims that she had been abandoned by Lomas as he looked for work in LA.