During opening arguments, Allison Coss's lawyer claimed that Coss and Stamos, now 46, had romantic encounter when Coss was a high school student on spring break in Florida in 2004, the Associated Press reports.
Stamos attorney William Sobel later said in a statement that "the allegations made today in the courtroom by the defendant's attorneys during opening statements will not be proven because they are simply untrue."
The judge in the case has already ruled that testimony about any romantic liaison will not be admitted, but the defense was permitted to mention it during opening statements. Instead, the trial's focus is on whether Coss and a friend, Scott Sippola, 31, tried to extort $680,000 from Stamos by telling him they had photos showing him with cocaine and strippers.
Prosecutors admit that Stamos and Coss met in Orlando in 2004 and subsequently exchanged e-mails, but say no incriminating photos exist.
Stamos contacted the FBI last fall after receiving two e-mails – one from a "Jessica T" claiming she was pregnant with Stamos's child, and one from a "Brian L" threatening to sell allegedly compromising photos to the tabloids. The defense admits that Coss and Sippola tried to sell photos to Stamos but said it was a legitimate business offer and not a crime.
Stamos attended the first day of the trial in Marquette, Mich., and is expected to testify.