Gwyneth Paltrow and Chris Martin reportedly agreed not to marry again until their children are adults.
The former couple - who have kids Apple, 11, and Moses, nine, together - split almost two years ago and it emerged last week the Coldplay frontman has yet to respond to divorce papers which he was served with in May last year.
It has now been claimed the Paradise hitmaker is deliberately "stalling" because he is concerned about how serious the Iron Man 3 actress' relationship with Brad Falchuk has gotten.
A source said: "The truth is, Chris stalling on signing documents is very much a calculated move to stop Gwyneth remarrying. He's just not ready to let her go.
"At the start, things with Brad and Gwyneth were very off and on but in recent months it's got serious and that hasn't gone unnoticed by Chris.
"When he and Gwyneth first agreed to consciously uncouple they completely prioritised their children, and one of the agreements was that they wouldn't remarry until the children were much older and had moved out of home."
Though 39-year-old Chris - who has dated actresses Jennifer Lawrence and Annabelle Wallis since splitting from Gwyneth - doesn't want to reconcile with his spouse, he isn't ready to see her finally move on.
The source added to Britain's Grazia magazine: "Gwyneth and Chris still share everything and she's openly spoken to him about the idea of her marrying - and even having children with - Brad.
"The problem with Chris is he doesn't know what he wants. He doesn't seriously want to get back with Gwyneth but he also isn't ready to see her settled down with anyone else."
- Bang! Showbiz