He tells The Sun that,
"Brit started DRINKING when she was a 13-year-old Mickey Mouse Club presenter and dabbled with DRUGS from the age of 14.
She is also terrified she will mimic her tragic grandmother Emma Jean, who committed suicide aged 31.
We tracked William - known as Willie - down to his camper van home near Britney’s childhood town of Kentwood, Louisiana.
After her turbulent early years, Willie says she went “buck-wild” on turning 18, snorting cocaine on her birthday, adding: “She has tried just about everything - cocaine, crystal meth, ecstasy.”
Willie, a welder, decided to speak out after seeing Britney’s capitulation since losing full-time access to sons SEAN PRESTON, two, and JAYDEN JAMES, one, in her bitter custody row with Kevin Federline.
Willie claims her recent meltdown can be traced back to her difficult past.
Not so innocent ... Brit on Disney
He says: “I’m not just a family friend or someone who claims to know them. I was there every day, working with them. I have seen all the terrible things happen.”
Willie says his niece’s battle with the bottle started when she was just 13.
On one occasion, he was invited to a Florida wrap party for Disney kids’ TV show Mickey Mouse Club - which Britney fronted alongside JUSTIN TIMBERLAKE and CHRISTINA AGUILERA.
He says: “The kids were definitely drinking that night. While the adults talked, they were somewhere else, up to no good.
“They had a bottle of gin and slipped away to the bathroom to drink it. I saw them going.”
But Willie is certain Britney was exposed to drink and drugs long before she hit the big time.
“Her brother Bryan was the star of the high school football team. They went to all the best parties where there were drugs and alcohol - a lot.”
He believes her drug-taking started at 14 with marijuana but adds: “It got worse when she got famous and she found out that people would do what she wanted them to.
“She wanted to drink, party and go where she wanted to go.
“She is a ruthless little thing, make no mistake. Nobody controls her. She bosses everyone around her.”
Willie says his brother JAMIE - Britney’s dad - must take a large chunk of responsibility for her boozy lifestyle.
“Just about everyone in the Spears family is a drunk and Britney’s dad Jamie was one of the worst.
“He would drink from daylight to night. Britney saw him drunk all the time.”
Jamie went into rehab in 2004 to overcome an addiction to alcohol and painkillers - but the damage to his family was already done.
Willie continues: “Britney grew up thinking it was alright to drink and that you can live with a drinking problem.”
Willie is a recovering drug addict and has been clean for more than four years. But sensationally he admits taking drugs with Britney in the past.
“If people think she has reached the bottom, they are very wrong. Suicide is the bottom line for Britney.”