COPENHAGEN — Peter Madsen, a Danish inventor, will not appeal his conviction for killing a journalist aboard the submarine he built, prosecutors said, in a new twist to one of the most-watched — and most gruesome — criminal cases in recent European history.

Mr. Madsen will appeal the life sentence a court imposed on him — a rare penalty in Denmark, even in murder cases — but he will not challenge the verdict delivered on April 25 that he was guilty of sexually assaulting and killing Kim Wall, and desecrating her body. “The appeal case will only concern the sentencing, and not the issue of guilt,” the Copenhagen District prosecutor’s office wrote on Twitter.

Peter Madsen’s lawyer, Betina Hald Engmark, said in a text message that the move was “certainly no recognition” of guilt.

Even with a life sentence, Mr. Madsen could be considered for release in as little as 12 years.

Ms. Wall, 30, who was Swedish, was last seen alive on Aug. 10, when she boarded the vessel, planning to interview Mr. Madsen, 47. Mr. Madsen, who had designed submarines and founded a company to build spacecraft, sank the sub and was rescued the next day.