Stuart Lubbock, 31, was found dead in Barrymore’s swimming pool in March 2001 after a party at what was then the entertainer’s home in Roydon in Essex.
Lubbock had sustained an array of internal injuries which indicated he may have been sexually assaulted before his death, but no one was ever charged in connection with the murder.
He had also taken ecstasy, cocaine and alcohol before he died.
According to the report, published by the Independent Police Complaints Commission (IPCC), investigating officers called to the site of the murder did not effectively preserve the scene and allowed unauthorised people to remain in the area.
A pool thermometer and a door handle, which may have been used to assault Lubbock before his death, were never seized as evidence nor forensically tested, it found, and several blood-stained pieces of clothing were not promptly investigated.
Police failed to “promptly investigate” blood left on the victim’s boxer shorts, towels and a robe.
The IPCC upheld six of 36 complaints and allegations made by Stuart’s father, Terry Lubbock.
The findings prompted Essex police to offer an unreserved apology to the murdered man’s family and friends.
The IPCC opened its inquiry in 2007 in response to complaints filed by Terry Lubbock, who accused officers of professional incompetence.
“There were not that many errors... But the errors that were made were quite significant,” Lubbock said yesterday, vowing to continue the fight for justice for his dead son.
He added: “If they had done the job properly in the first place we would have had people in court charged. I am sure of that.
“And that is what I am still pursuing. I want justice to be done and I will not rest until it is.”
IPCC commissioner David Petch said there was “no grounds” to support allegations of police corruption, while stressing that only some aspects of the investigation were “incompetent”.
“We will probably never know whether the missing thermometer and door handle were evidentially important, but not securing these items was a failure and leaves questions unanswered.”
In June 2007, Barrymore and two other men were arrested on suspicion of sexual assault and murder but later released without charge.
Essex Deputy Chief Constable Andy Bliss said he had apologised to Lubbock senior.
“I have visited Terry Lubbock to deliver a personal apology on behalf of Essex Police and will also be speaking to other members of his family,” he said.
Last night, Barrymore’s solicitor, Henri Brandman, said his client “has always protested his innocence in respect of the matter”.
“That remains the case,” he said.
“Neither Michael nor I will be making any further comment.”