Singer Gwen Stefani divorced Gavin Rossdale because he was cheating on her with Mindy Mann, a nanny for their three children, according to a source.

A source told eonline.com on Thursday that the "Hollaback girl" singer learnt about her ex-husband's affair by discovering sex chats linked to an iPad for one of their children.

A day after attending the 2015 Grammy Awards alone, "Gwen's suspicions were confirmed", said the source.

"One of the other nannies went to get the iPad for one of the boys and found texts and photos that Mindy had sent to Gavin. Gavin's texts were linked to the iPad and could be seen. They talked about meeting up to have sex and she sent naked photos. The nanny told Gwen everything," the source added.

After discovering the evidence of Gavin's affair, "(Gwen) immediately confronted Gavin and he tried to deny everything once again".

"He had a story that he was just flirting but this time Gwen had seen it with her own eyes and didn't believe it. She couldn't get past it and was devastated this was going on right under her nose. She asked Gavin to leave and knew the marriage was over," the source added.

Stefani filed for divorce from Rossdale in August after 13 years of marriage.

The 46-year-old star has sons Kingston, nine, and Zuma, seven, with her estranged husband.