According to this week's heat magazine, the pair argued over the reality talent show after Tulisa joined the series last summer.
A friend of the brunette beauty told the publication that Fazer 'disapproved' of her new career.
"He told her that he thinks she should quit The X Factor as she is better than that and is giving the band a bad name," the pal explains.
"It really cut through Tulisa and she started to cry. She now says that at that moment, she knew their relationship was dead."
And it sounds as though the split between the pair leaves N-Dubz' future as a band in doubt.
The friend adds: “She realises they are in a band together and have to sort out their problems but she isn’t one to just get over things like this.”
Tulisa and Fazer ended their 18-month relationship earlier this month, after a "make or break" Christmas holiday failed to reconcile the couple.