Twilight heartthrob couple Robert Pattinson and Kristen Stewart were reportedly in an open relationship from the day they started seeing each other.

According to a Radar Online report, the two had an agreement that they could see other people while in the relationship.

Stewart made headlines in the summer of 2012 when she was caught cheating with her Snow White and the Huntsman director Rupert Sanders. Her liaison with the 43-year-old was uncovered when pictures of the father-of-two kissing the actress in public were published in Us Weekly.

"When they were officially dating, they always had a policy of 100 percent honesty, which did create issues. Robert was always very frank about his flings with other women and that worked for Kristen too," the alleged insider told the website.

"They had an open relationship and it's amazing that Kristen hasn't revealed this info when she was being painted as a 'jezebel.' Rob has had other women on the side and Kristen has had other women and men as well," the source added.

Adding on, the Radar Online claims that the couple took a "bizarre approach to relationships" and it was more business than romance.

"Their relationship was always a business and a spiritual partnership more than a romantic one. Rob and Kristen often said they might never see each other again after the last Twilight press tour and that's pretty much what happened, except for their 'are you in town' booty calls," the source went on.

"They were pretty up-front with each other when they got sick and tired of each other. It was the strangest relationship, but keeping each other at arm's length is working in their favour," says the insider. "Kristen and Rob are still close and definitely in contact, and they still sleep together when the opportunity comes up."