The 33-year-old Cilla star and Greg, 36, allegedly started dating just three months after he split from the mother of his three children, Catherine.
Sheridan and Greg were reportedly spotted kissing at a restaurant last week.
One diner at Smuggler’s Cove in Liverpool claimed on Twitter: “Had a lovely night at @smugglerscoveL for rea 18th my fav place seems that @Sheridansmith1 was hungry to she nearly swallowed her partner [sic]”
Despite Greg and Sheridan’s involvement starting after her separation from him, Catherine is said to be cut up.
A source told The Sun: “Catherine was devastated when she realised that Greg was dating Sheridan so soon after they split.
“She found out in November that they were sending each other flirty messages on Twitter, but didn’t know things had gone any further.”
However Greg has taken to Twitter to deny any relationship with Sheridan, insisting he and his ‘ex’ split up last summer.
He said: “I am single, my ex and I separated last summer and are currently going through a divorce.”
Sheridan has not commented on the reports.