A 50-year-old man has been arrested on suspicion of the indecent assault and murder of Stuart Lubbock at the home of Michael Barrymore.
Mr Lubbock, 31, was found dead in the TV presenter’s pool in Roydon, Essex, in March 2001.
The man was arrested in Cheshire and is being questioned in custody, Essex Police said.
Mr Lubbock had been attending a party at the home of Mr Barrymore, 68, with eight other people.
A post-mortem examination found Mr Lubbock had suffered “horrific” injuries prior to his death, police said.
Alcohol, ecstasy and cocaine were found in his bloodstream.
Det Supt Lucy Morris, Essex Police’s head of major crime, said the arrest was made after “significant new information” came to light following a renewed appeal in February 2020.
“This information has led to us making an arrest and over the coming days we will be contacting all those who were present at the party at the time as well as others who may have information,” she said.
“As we stated last February and have continually stated over the last 20 years, we believe someone or some people at that party know what happened.”
Det Supt Morris said the investigation had been “extremely lengthy and complex”, adding: “We have never given up on finding out what exactly happened to Stuart and we will not stop in our pursuit of justice for him and for his family and friends.”
Mr Lubbock, a butcher from Harlow, was described by neighbours at the time of his death as a “pleasant, sociable” man.
No-one has ever been charged with any offence in relation to his death.
Reacting to news of the arrest, Mr Lubbock’s father Terry said: “There is just so much going on in my head. I can’t get my head around it.
“Of course I’m happy. Of course this is good news. But it’s been 20 years. This has nearly killed me.”
In February, the 76-year-old from Harlow said he had only months to live after being diagnosed with terminal cancer.
“All I’ve been living for is to get justice for Stuart,” he said.
Mr Barrymore and two other men were arrested in 2007 on suspicion of sexual assault and murder, but were later released without charge.
The TV personality, who was known for shows including Strike It Lucky, has always denied involvement in the death.
Speaking last year, he said: “I have had nothing to do with this whatsoever and yet I keep getting bashed and bullied by the media.”
An inquest into Mr Lubbock’s death, in 2002, recorded an open verdict.
Three years ago, the then-attorney general, Jeremy Wright, refused to let Terry Lubbock make a High Court application for a second inquest.
In 2009, the police watchdog published the findings of a review of the investigation into Mr Lubbock’s death, concluding that officers missed crucial evidence and did not ensure vital forensic tests were completed until six years later.
Last year’s police appeal coincided with the broadcast of a Channel 4 documentary about Mr Lubbock’s death.