Midlands property tycoon John Broadhurst, 40, had been charged with 26-year-old Natalie Connolly’s murder.
But after four weeks of evidence at Birmingham Crown Court the judge ordered the jury to return a not-guilty verdict following the crown’s acceptance of the lesser charge of manslaughter.
The victim was found at the couple’s detached house, in Kenrose Mill, Kinver, near Stourbridge, with more than 40 injuries after a night of sado-masochistic sex games on December 18, 2016.
She had also consumed potentially fatal levels of alcohol and also cocaine.
A post mortem found that she died from a combination of the injuries and alcohol.
Broadhurst claimed he had tried to lift her up the stairs that night but when he was unsuccessful he left her there and went to bed.
He found her in the same spot the next morning at 9.25am and only then did he call an ambulance.
Broadhurst claims he discovered Ms Connolly, also known as Andrews, at the foot of the stairs of their home the morning after their night of ‘rough sex’.
The manslaughter charge accuses him of killing Ms Connolly through gross negligence by failing in his duty of care as her partner to call the emergency services, and it was this failure that led to her death.
As a result of the guilty plea, the trial was halted and sentencing will now take place next Monday.
Mr David Mason, QC, prosecuting, said that it was felt that this was the appropriate way of proceeding in a ‘very complicated’ case and it was also the wish of Ms Connolly’s family.
Explaining the Crown's decision to accept Broadhurst's plea to a lesser charge, the prosecutor told the court: "The evidence in this case, which is an extremely unusual case, has been very complicated.
"The family (of the victim) have been consulted and are entirely content with the view that the prosecution have taken."
Broadhurst, a property developer said to have a fortune of around £15 million, had separated from a former partner, who was pregnant with their second child, around three months before Miss Connolly's death.
The jury had been told earlier in the trial that the pair spent the day drinking with friends while watching a West Bromwich Albion game at The Hawthorns before going for a curry on December 17, 2016.
The couple, who both have children from previous relationships, took their main courses back to their home in the defendant’s Range Rover driven by a chauffeur.
They consumed cocaine in the kitchen before the victim indicated she wanted ‘masochistic sex’, which they were both interested in, Mr Mason said earlier in the case.
The court heard that Miss Connolly, who had suffered a "blow-out" fracture to her left eye, bruising and internal injuries, had told witnesses that she and the defendant had an interest in masochistic sex.
A paramedic told Broadhurst's trial that the father-of-three appeared hungover and did not seem "unduly upset" after finding his partner's body at about 9.30am.
Broadhurst, of Wolverley, near Kidderminster, will be sentenced next Monday after being granted bail.
Adjourning the case, trial judge Mr Justice Julian Knowles told Broadhurst: "You have pleaded guilty to manslaughter.
"Whilst it is not a charge of murder, it is nonetheless an exceptionally serious offence. All sentencing options remain open. You should be prepared for a custodial sentence of some length."
Following her death devastated friends and family paid tribute to their 'angel' Natalie.
Natalie's brother, Jason Andrews, posted a touching tribute on Facebook which read: "Rip my beautiful sister love u so much," while her mother, Karen Whittington Connolly, added: "My heart my life my soul."
Neighbour Brian Davies said the couple were renting the property while their house in nearby Chaddesley Corbett was being built.
The retired journalist said the couple had been due to fly to Dubai to spend Christmas there before she was found dead.