Jamie Lynn Spears' fiancé Casey Aldridge reportedly "instigated" an affair with a 28-year-old woman while Jamie Lynn was expecting the couple's first child.

The 'Zoey 101' star – who gave birth to daughter Maddie Briann in June – is said to have been oblivious that her 19-year-old husband-to-be was cheating on her with Kelli Dawson.

Dawson said: "He totally instigated it and pursued me."

However, Aldridge slowed things down with Dawson as Spears neared the end of her third trimester, because he was so overcome with guilt.

Dawson added to In Touch Weekly magazine: "Casey had to deal with the baby coming, and it had to stop. But even after we stopped having sex, we would still kiss occasionally.

"I still see him all the time, at least once a week."

The pair's relationship has also been confirmed by Jason Alexander, who was famously married to Jamie Lynn's older sister Britney for 55 hours after a Las Vegas wedding in 2004.

Alexander said: "Kelli and Casey have been a couple on and off for a few years. They were a really tight couple, but I think it was hard for Kelli, having Jamie Lynn in the picture."