The US socialite - who is pregnant with his first son - is upset with how close the media mogul is to his ex-girlfriend.
A source told The Sun on Sunday newspaper: ''Lauren doesn't want Simon near Sinitta anymore.
''And as difficult as it is for him, Simon is going to back the mother of his baby. Simon tore a strip off Sinitta - but then he pleaded with her to see sense and keep a low profile.
''But she's being driven insane with jealousy and isn't able to control herself.''
The reports come after Sinitta took to Twitter to promise she would speak the truth about her abortion of Simon's baby a few years ago, and friends are said to be concerned she is heading for a breakdown.
Speaking to the Daily Star Sunday newspaper, an insider said: ''This isn't the way Sinitta saw it happening in her head.
''The news had knocked her completely and she hasn't been in a good way since she found out. Simon always told her he didn't want children.
''She didn't think he would ever start a family and believed if he ever changed his mind, he would start a family with her. She is finding it hard to control her emotions and friends are worried about her, they think she is having a breakdown.''