The ex-Hollyoaks actor allegedly invited carer Katie Moores back to his hotel room after meeting her in Leeds, says The Sun.
Moores told the paper that Lomas said "hardly anything" about Marsh during their time together, nor did he mention their 1-year-old daughter Polly.
Moores, who reportedly lists "Jamie Lomas" among her interests on Facebook, claimed that he invited her and two other girls to the city's Park Plaza hotel for drinks after they met on a night out.
She said: "At one point he did say he was single. My friend knew he was with Kym but I didn't.
"He didn't really say much about Kym - only that they'd just got back off holiday. He didn't want to talk about her."
She added: "The kiss wasn't anything like a passionate kiss - just a stupid drunken one.
"We were sat next to each other on the bed. It just happened. I don't remember who started it. It wasn't very long, probably about 20 seconds.
"After that we were just messing about, play fighting."
Marsh, who plays Michelle Connor in Coronation Street, recently laughed off claims that her relationship with Lomas is on the rocks and that she doubts his prospects of finding work in Los Angeles.
> Kym Marsh 'annoyed' over Jamie Lomas relationship crisis claims
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