Coleen Rooney falsely accused Rebekah Vardy of a "clear betrayal of trust" for allegedly leaking stories about her private life, the High Court in London has heard.
Ms Rooney, 34, was dubbed "Wagatha Christie" on social media for her apparent sleuthing work, having "posted a series of false stories to see if they made their way into The Sun" as part of a "sting operation".
The wife of former England star Wayne Rooney claimed in October 2019 that fellow footballer's wife Ms Vardy, 38, shared the fake stories she had posted on her personal Instagram account.
Ms Rooney wrote: "For a few years now someone who I trusted to follow me on my personal Instagram account has been consistently informing The Sun newspaper of my private posts and stories."
She dramatically added: "I have saved and screenshotted all the original stories which clearly show just one person has viewed them.
"It's ................ Rebekah Vardy's account."
Ms Vardy, who is married to Leicester City striker Jamie Vardy, has denied the accusations and is suing Ms Rooney for damages for libel.
At the first hearing of their high-profile legal battle, Mr Justice Warby is being asked to determine the "natural and ordinary" meaning of Ms Rooney's posts on Instagram and Twitter.
Ms Vardy's barrister Hugh Tomlinson QC said Ms Rooney's posts were an "untrue and unjustified defamatory attack" which was "published and republished to millions of people".
He said Ms Rooney's accusation "leaves the reasonable reader in no doubt that the defendant is accusing the claimant of consistently and repeatedly betraying her trust over several years".
He added: "In fact, she did nothing wrong. Whatever leaks there were did not come from her."
Mr Tomlinson said Ms Rooney's post featured "a build-up to the 'big reveal' of the identity of the person responsible", adding: "The description of the defendant's detective work makes it clear that the claimant is being said to be responsible for the leaking of the three specific stories mentioned."
Ms Rooney claimed Ms Vardy leaked three "false stories" she posted on Instagram about "gender selection in Mexico", Mr Rooney returning to TV, and the basement in her new house flooding.
Ms Vardy's lawyers argue that the meaning of Ms Rooney's posts was that she had "consistently and repeatedly betrayed the defendant's trust over several years by leaking the defendant's private and personal Instagram posts and stories for publication in The Sun".
Ms Rooney's legal team says the posts mean "there are reasonable grounds to suspect that the claimant was responsible for consistently passing on information about the defendant's private Instagram posts and stories to The Sun newspaper".
David Sherborne, representing Ms Rooney, said in written submissions: "It is true that the claimant was responsible for consistently passing on information about the defendant's private Instagram posts and stories to The Sun, and that this was part of her history and practice of providing private information to the press, especially The Sun, with whom she had a very close relationship."
He said Ms Rooney's post accusing Ms Vardy of leaking the stories "followed earlier warnings which the defendant had been forced to post publicly, as well as privately, as a result of information from her private social media accounts being leaked to The Sun".
He added: "The defendant was so concerned that she even posted a number of fake stories on her private Instagram account in order to discover where the leak was coming from."
Mr Sherborne told the court Ms Rooney conducted a "sting operation" to find out the source of the leaks, and "deliberately limited the accessibility to this private account and the sting operation stories" to just Ms Vardy.
He said: "The fact that these sting operation stories also then appeared in The Sun, after access to her private account was limited to just the @beckyvardy account, is the reason why the defendant published the post which is the subject of this claim.
"The impression the reader would take away would be the essential message, that it was Rebekah Vardy's account that was the source of private stories about the defendant appearing in The Sun - not Rebekah Vardy herself.
"The impression the post gives the ordinary reader stops short of guilt."
In her written claim filed with the court, Ms Vardy said she had worried she would lose her baby because of the stress as she was pregnant at the time.
She also "suffered extreme distress, hurt, anxiety and embarrassment as a result of the publication of the post and the events which followed", her lawyers allege.
Neither Ms Vardy nor Ms Rooney attended the hearing at the Royal Courts of Justice in London.