It says Federline wants more than $30 million from Spears, along with custody of the couple's two young children, Sean Preston, 1, and Jayden James, 8 weeks. The newspaper also reports that Federline has been offered $50 million for the tape by one film company that wants to put the video online.
The tabloid says Spears fears release of the tape would short-circuit the comeback she's attempting to stage.
But on The Early Show Tuesday, Us Weekly magazine's contributing editor Katrina Szish said Spears probably needn't worry.
"If a tape does exist, I don't think it will be damaging for Britney," Szish told co-anchor Rene Syler. "We've seen her at her lowest point. She's on her way back up. This would be a tape she did with her husband when they were married. Whatever may be in that tape, it really doesn't matter anymore.
"At this point, (the existence of the tape is) pure speculation … It's completely hearsay, so we can't necessarily yet say that Kevin is doing anything wrong."
And if Federline is indeed using such a tape the way the tabloid says he is, it is likely to backfire, Szish said, adding: "If this sex tape is real, if it does exist, if he is trying to demand money for it, that would certainly hurt his chances (with a judge) of getting sole custody (of the children). So, I do think it's probably unlikely that the sex tape does exist."
Separately, a judge in Los Angles last week threw out a Spears lawsuit against Us Weekly, ruling the pop star cannot be defamed by the magazine publishing rumors that she and Federline had made a sex tape and were worried about its release.
Superior Court Judge Lisa Hart Cole said Spears has "put her modern sexuality squarely, and profitably, before the public eye" and it would be unlikely for the magazine article to be found defamatory.
The judge's decision to dismiss the $10 million lawsuit filed last year did not address whether the October 2005 story was true or false.
"The issue is whether it is defamatory to state that a husband and wife taped themselves engaging in consensual sex," Cole wrote in the decision issued last week. "The backdrop against which this issue must be addressed is that the plaintiff has publicly portrayed herself in a sexual way in her performances, in published photographs and in a reality show."
Spears' lawyer, Martin Singer, did not return a call Monday seeking comment.
Us said in a statement it stood by its reporting and was pleased with the decision.
The lawsuit stemmed from an article published Oct. 17, 2005, in the magazine's "Hot Stuff" column under the headline, "Brit & Kev: Secret Sex Tape? New parents have a new worry: racy footage from 2004."
It claimed Spears and Federline feared the release of a secret sex tape, which they had viewed with their estate planning lawyers. The article said that Spears gave a copy of the tape to lawyers and she and Federline were "acting goofy the whole time" while watching the video.
"There was no laughter, disgust or goofy behavior while watching the video in the company of lawyers because they did not watch any video, and because there is no such video," the lawsuit said.
Spears sued after Us refused to issue a retraction.