And he says he is ready to reveal the explosive truth about the star’s background and her tearaway years as a young musician.
The news will be a blow to Tulisa Contostavlos who is on bail over unrelated allegations that she fixed an £800 cocaine deal for an undercover reporter.
She has been holed up in her £6million Hertfordshire mansion after being quizzed by police following alleged boasts that half her phonebook sold Class A drugs.
A source said the housemate, who has been involved in music production, is now a professional sportsman.
The contact added: “Tulisa Contostavlos should be very worried. This guy has known her, Dappy and Fazer for years and used to write music with them when they were all in their teens.
“He is very well-placed to know a lot about some of the darker side of Tulisa Contostavlos’s character. This could be massive for her.
“I guarantee you Dappy will be watching Big Brother to see this guy as they were so close they even had nicknames for each other.”
Tulisa Contostavlos, 24, has always traded on her gritty street image, telling how she dropped out of school at 15 without a single GCSE.
She joined a girl gang, running wild on the streets of London and helped other members snatch handbags. The singer blamed her tough upbringing on the stress of having a mentally ill mother and a dad who could not cope with her illness.
She said that, despite living in posh Belsize Park, “most of my early childhood memories are not happy ones”.
Tulisa Contostavlos’s arrest hell comes after a nightmare 12 months for her.
In March 2012 a video surfaced showing the singer engaged in a sex act with an ex-boyfriend, rapper Justin Edwards.
It was loaded on to the net, forcing Tulisa Contostavlos to obtain an injunction from the High Court banning the film from being circulated.
She later spoke out saying she was “heartbroken” to be betrayed by someone she once loved.
Tulisa Contostavlos has also been dropped from The X Factor by boss Simon Cowell, 53, and an £8m deal to star in a Bollywood film was recently exposed as a sham.
The source added: “Things probably couldn’t get worse for Tulisa Contostavlos, unless this guy spills the beans.
“He is a real way in to understanding what the N-Dubz guys were like when they were younger, and some of it ain’t pretty.