Michael Mannion, 35, known as Masala Mike, today told how he was left "shocked and appalled" after witnessing his friend Samson D'Souza attack 15-year-old Scarlett.
Mannion, a carpenter from London, is to be questioned by police in Goa's Porvorim station following his decision to come out of hiding and hand himself in.
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Witness: The British man known as 'Masala Mike', 35-year-old Michael Mannion, speaks to BBC News Today about how he saw Scarlett Keeling being assaulted by his friend in a Goa holiday shack
The 15-year-old's battered and bruised body was found on Goa's Anjuna beach on the morning of 18 February.
Mannion was staying in Goa with D'Souza and his French wife Cecile after meeting the couple six years ago on a previous trip to India.
In a statement due to be handed to police he says he saw barman D'Souza lying on top of Scarlett trying to remove her clothing.
"I was shocked and appalled because despite my informing Samson D'Souza and the others in the shack that the girl was a minor, he was assaulting her," said Mannion.
Fearing D'Souza and his friends would kill him if he intervened, Mannion said: "I did not have the strength to physically confront them.
"I got on my scooter and drove away. But I was feeling rage at their actions against a minor girl."
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Schoolgirl: Scarlett was just 15 when she died
He added: "Disgusted and in rage, but feeling helpless, I then drove back to Samson D'Souza's house.
"The door was locked and I did not have a key to get in." Mannion waited until 6am - just half an hour before Scarlett's body was found - for D'Souza to return.
He was told later that day a body matching Scarlett's description had been found on the beach.
"I was keen to report what I had witnessed to the local police but was afraid for my own life. I had been threatened with dire consequences by Samson D'Souza," he said.
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Scarlett and a female friend soak up the sun while in Goa
Scarlett's mother Fiona MacKeown today welcomed Mannion's "brave" decision to give police vital evidence.
She said: "I am so glad that Mike has finally had the courage to come forward despite the danger to himself.
"He has more of an idea how big this is than even I have. He knows more about the danger of the drug scene in Goa than I would ever have begun to know.
"He had genuine reason to be afraid at the time and he did try to stop it.
"I think he feels very responible. He couldn't just go wailing into this, especially with all the people who are murdered in Goa with police knowledge."
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Holiday tragedy: British teenager Scarlett Keeling rides an elephant through a swamp just days before she was killed on holiday in Goa
After learning Goa police wanted to question him about the attack Mannion says he approached the British Deputy High Commission in Mumbai for help but was told it could not interfere in Indian judicial proceedings.
He left Goa soon after Scarlett's body was found. "In fear of being killed in case I deposed on the matter I kept my distance from the people I feared could kill me," he said.
Mannion said he had now come forward after the police promised he would be safe.
Mrs MacKeown's lawyer Vikram Varma added: "Mike thought his life was in danger.
"It is understandable that he was scared. He knew Samson could kill him or use his police connections against him."
Accompanied by Scarlett's mother Fiona MacKeown and her lawyer, Mannion said he remained scared "people from Anjuna" would try to harm him if he gave evidence.
He was also afraid police would try to implicate him in the crime.
"It is important for me to be here with Fiona today because justice needs to be done," he said.
"At the end of the day a 15 year old girl has been raped and murdered.
"That is a terrible crime wherever it happens, anywhere in the world.
"I have suffered through this, but we must remember that Fiona and her family have suffered the most. They have lost a daughter and a sister."
Mannion had been friends with D'Souza and his French wife Cecile for six years after meeting them on a previous holiday in India.
In a statement prepared for the police he said he saw his friend on top of Scarlett on the beach trying to remove her clothes shortly after 5am on February 18.
Scarlett's bruised and battered body was found less than 90 minutes later. She had been raped.
Mannion fled Goa afraid for his life after being threatened with "dire consequences" if he reported what he had seen.
Asked if he was still afraid for his life, Mannion said: "I am."
He added: "Initially I was advised to stay away from the state.
"I finally came back when I felt it was safe for me when I had the backing of the British Console and [Ms MacKeown's lawyer] Mr. Varma."
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