Blood and items left near the body of a man found dead by Michael Barrymore's swimming pool were not properly investigated, a report has found.
Stuart Lubbock, 31, died at the entertainer's home in 2001.
The Independent Police Complaints Commission (IPCC) inquiry noted two pieces of evidence - a pool thermometer and a door handle - were not seized.
Police also 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.
No one has ever been charged in connection with the death of Mr Lubbock and Mr Barrymore has always denied involvement.
Post-mortem tests showed Mr Lubbock suffered internal injuries which suggested sexual assault.
He had also taken ecstasy, cocaine and alcohol before he died.
A coroner recorded an open verdict into Mr Lubbock's death following an inquest in September 2002.
The IPCC report revealed that the two pieces of evidence which were not seized by police and subsequently went missing could have been crucial to the case.
The swimming pool thermometer appears in photographs of the scene but, along with the detached door handle, was never forensically tested.
It has not been possible for the police to establish whether or not the implements were used to cause Mr Lubbock's injuries, the IPCC said.
The complaints which were upheld include: the scene not being effectively preserved; unauthorised people allowed to stay at the scene; a member of the public being allowed to take the temperature of the swimming pool, and the investigation being suspended prematurely.
IPCC Commissioner David Petch said: "There are absolutely no grounds to support allegations that officers acted corruptly.
"The view that the entire investigation was incompetent is not borne out by the bulk of the evidence.
"But undoubtedly there were failings in some aspects of the investigation.
"As a consequence there are lingering fears that, because the integrity of the scene was not properly preserved, important evidence may have been lost."
He added: "In our view the decision in December 2001 to suspend the investigation was premature - at that time some key forensic work had not been completed and some inquiries were still outstanding.
"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.
"All of these shortcomings must be frustrating and distressing for Mr Lubbock, who has worked tirelessly to find out what happened to his son."
Terry Lubbock said the IPCC report revealed "significant" police errors.
He said: "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."
The IPCC began its inquiry in March 2007 after receiving complaints from Mr Lubbock.
Deputy Chief Constable of Essex Police Andy Bliss said he had visited Mr Lubbock to deliver a personal apology on behalf of the force.
"Over the last eight years, Essex Police has taken a number of steps to improve investigations into suspicious deaths.
"Since this investigation, Essex Police has introduced specific training for dealing with suspicious deaths and we continually seek to improve this.
"We remain determined to find out what happened to Stuart back in March 2001. A new investigation began in 2006 and this remains ongoing."