Tom Cruise and Katie Holmes' marriage was allegedly affected by his relationship with Scientology leader David Miscavige.
Miscavige - who heads the sci-fi cult, of which Cruise is a devotee - was best man at the couple's wedding, and sources have claimed their intense friendship is one of the reasons they grew apart.
A source told The Sun newspaper: "Miscavige and Tom are joined at the hip. Tom runs every important decision in his life by him.
"Tom really values his opinion and Katie often felt like she came in second place. When it became clear how much influence he had over her husband she knew she had to get away."
Miscavige is said to have given many lavish gifts to Cruise, including an aircraft hanger for his private jet and a custom-built luxury coach.
Former Dawson's Creek actress Holmes, 33, filed for divorce from Cruise last Thursday, and one of her reasons is because she feared their six-year-old daughter Suri would be heavily influenced by Scientology if she stayed in the relationship.
Insiders say Cruise - who has been married three times - moved away from Scientology during his union with second spouse Nicole Kidman, but when they split in 2001 he turned to the organisation for guidance.
A source said: "Nicole was really sour on Scientology and kept pressing Tom to stay away.
"She spotted Miscavige and didn't like what she saw. Her big issue was that Tom was becoming like Miscavige. Nicole didn't like the relationship that was developing between them."
Holmes was raised a Christian, but did partake in the Scientologist practice of 'Auditing' - a process which sees an individual confess their life experiences to an auditor in a bid to free themselves from unwanted psychological barriers - although "she was never completely committed to Scientology, but participated because she was in love with Tom and knew it meant a lot to him".
Cruise flew back from Icleand to the US on Tuesday to celebrate his 50th birthday.
Holmes is willing to "play tough" against Cruise.
She has hired top lawyer Allan E. Mayefsky since filing for divorce last week as she is determined to get sole custody of Suri.
Well-known legal analyst and author Lisa Bloom told website The Daily Beast: "She's making it clear she's going to play tough. Cruise is more of a high-profile celebrity than she is, so he has a greater distance to fall if grievances are aired. Everyone has dirty laundry they don't want aired. She's saying play fair with me or else.
"Of course she would choose an experienced and tough lawyer. But the fact that she'd choose someone who's been known to go to media in the past signals that she might have hired him for that reason."
Bloom believes Holmes - who filed for divorce in New York, where courts are more likely to grant custody of a child to one parent - needed to hire someone tough like Mayefsky as he will be able to compete with Cruise's legal team.
Bloom - who was on the receiving end of Mayefsky and his team when she divorced Phillip J. Smith, chairman of the Shubert Organisation - said: "Katie needs cutthroat lawyers because Tom is really powerful. The dirtier the better, so I think she made a good choice. They are known for being ruthless, and they normally win. They beat the sh**t out of me. They make you out to be the biggest piece of trash there is. It took me a long time to get over it.
"Katie has really good, nasty, horrible lawyers and she should do really well."