We can reveal that Kevin Federline feared his ex would use the Beretta pistol he bought her as a birthday gift to MURDER their two young sons.
And it was his desperate call for help that started the dramatic three-hour siege at her Hollywood home as she held armed cops and a specialist SWAT team at bay and refused to hand over the boys.
Britney's latest problems come after a series of shocking incidents involving her which have been caught on video
The News of the World can reveal that K-Fed, 29, panicked on Thursday night after realising the fallen pop princess kept the handguna Beretta 92FSin the master bedroom where she was holding the kids, Jayden James, one, and Sean Preston, two.
Convinced that in her out-of-control state she would use the weapon to kill the lads then turn it on her herself, the wannabe rapper rang his lawyers who alerted the authorities.
Last night as Britney was sensationally released from hospital after throwing a tantrum, a source close to the warring couple revealed: "Kevin knew she was on the edge and might snap at any time.
"Knowing there were TWO firearms in the house, including the Beretta, he wasn't taking any chances hence the massive police response."
In the most nightmare week yet of Britney's crazed life, we can also reveal that although NOT high on illegal drugs she:
SWALLOWED bottles of hillbilly heroin'a highly-addictive drug called OxyContin that's killed hundreds in America,
BINGED on a mindbending cocktail of more precription drugs before defying the police,
PLEADED with Kevin to SELL her the kids for a multi-million payoff in a tearful phone call,
FOUGHT with bodyguards to hang on to her children,
BASHED her own head against the wall in a fit of blind rage.
Agonised
Britney, embroiled in a custody battle with estranged husband Kevin, was due to hand over the children to him on Thursday evening.
But K-Fed's mind started racing when the fallen star made an agonised call refusing to give them back and offering to BUY them.
Our source told us "Britney said that Kevin could have an incredible 100 million dollars if he'd give HER the kids and never see them again.
"He's used to Britney going off on one, but he knew from her voice that this was a whole other thing entirely. He hung up straight away and called his legal team in a panic.
"He told them that Britney was capable of anything and admitted that she still kept two handguns at the housethe pistol he bought her for a birthday gift and a black automatic Glock. The lawyers called the police immediately."
Before cops arrived at about 8pm Britney made a SECOND chilling phone call, predicting she was about to hit headlines worldwide.
She rang 16-year-old sister Jamie Lynn, who has been at the centre of a media storm since announcing she was pregnant, two years below the age of consent in California.
Our source said: "Jamie's new-found publicity enraged jealous Britney. And after calling Kevin that night she rang Jamie and told her, You're not going to be the only f***ing Spears on the front cover of a magazine next week!' and then hung up." Meanwhile Britney lost it when Kevin's lawyer Mark Kaplan showed up at the house and tried to talk her round.
Our source revealed: "Britney was wailing and sobbing uncontrollably then she just flipped. Her hysteria went up a notch and she ran with the children into walk-in wardrobe and slammed the door behind her.
"She sat in there sobbing and yelping out loudly. She was in there for about an hour refusing to come out. The children were also sobbing because they were so scared.
"A court-appointed monitor, who supervises the kids' visits, was also there but Britney refused to budge and could barely utter a word, just mumbling responses.
"When the cops, paramedics and K-Fed's bodyguards turned up with loads of commotion it sent Britney over the edge again.
"She was squealing out wildly, urging them not to come into her room. After a while the cops just about calmed her down and reasoned with her to let them in.
"When she opened the cupboard door she was sat cross-legged on the floor weeping uncontrollably. She was wiping her eyes and smearing make-up over her face. Her hair was all over the place and she looked pale and despondent.
"Her nose was running and she was dribbling like mad, shaking her head and shivering. She looked very flustered.
"She had one boy in each arm, partly rested on her lap, and they were crying their eyes out hysterically. They were beetroot red in the face, and their eyes seemed swollen with all the tears.
"She kept blubbing, and you could barely hear her say, Don't take the boys.' Eventually a police officer, the monitor and one of the bodyguards got close enough to reach out for the children. At first Britney let go of the boys and the team levered them out of her grasp.
"But once away from them she pounced on the bodyguard's leg and tried to stop him walking away. The cops tried to gently peel Britney from him but she started lashing out pleading, No! No!' She was clambering up his legs to get to Sean Preston, who began crying again. She was in a real state, sobbing then screaming at everyone.
"Then she sank her teeth into the guard's leg. It didn't really work as she got more of his jeans than his flesh. But she was acting so crazed. She got up on her feet and tried to kick the minder, who'd now passed the child on to a colleague. The paramedics then grappled with her and tried to calm her down.
"They sat her on the side of her bed but she just sank to the floor and crawled as quickly as she could across the room. As she sank her hands into the floor her nails broke off. She was screaming and yelling out for the kids.
"Then she stopped and just broke down in floods of tears againrocking from side to side and crying like a heartbroken child. That's when the paramedics persuaded her to lie down on a stretcher.
"But she was acting so wild they tied her in and decided they had to take her in for drugs and mental health checks.
"Even though the kids had been moved away by then she was still grabbing out for them."
Within minutes news pictures of the troubled star strapped to a stretcher and being carted off to the Cedars-Sinai hospital had flashed around the world. It was widely assumed Britney was on illegal drugs when she was taken but police sources told the News of the World:
"Tests revealed she was totally clean. She wasn't high on illegal substancesshe was crazy."
Los Angeles Police Department officer Ana Aguirre confirmed: "There was no actual crime involved. Police resolved the conflict. Both children were turned over to their father."
A source told us Britney's behaviour had recently become increasingly bizarre. The insider said: "About three weeks ago she started bashing her head against the wall when she's frustrated or annoyed about something.
"That's a form of self-harm. One of her team saw her doing it but never thought to intervene. She just thought it was Britney having one of her moments.
"Even when little Sean started copying her nobody did anything about it."
Yesterday the Toxic star woke screaming in hospital. Doctors rushed in to sedate her as she thrashed around yelling: "I want to kill myself! I want to die!" An inside source told us: "She was so agitated she was arching her back and pulling at the straps holding her down.
"She was like a woman possessed. It was like watching The Exorcist. It took two nurses to hold her down while doctors brought her under control."
But in a shock move last night she was released with no warning after a tantrum in which she SCREAMED at staff, RIPPED out her intravenous drip and DEMANDED to be allowed home.
Doctors agreed to the early release on condition she was signed over into the care of her father Jamie, 55, a reformed alcoholic and one-time builder.
The pair have had a rocky relationship but in recent weeks have grown closer.
A source said: "Jamie's an addict so feels a huge amount of guilt at the way his daughter has turned out. He thinks it's his fault as she must have his genes.
"So he has pledged to do everything in his power to help her get better."
Britney took 100 pills in 36 hours
CRAZED BRITNEY swallowed a deadly cocktail of more than ONE HUNDRED prescription tablets in the 36 hours before her custody meltdown, the News of the World can reveal.
And the singer washed them all down with a Purple Monster a mind-blowing home-made cocktail of vodka, American Nyquil anti-flu syrup and an energy drink like Red Bull.
A long-term friend of the star told us: "It looks like everything in her medicine cabinet went down her neck. It seems she was only saved from falling into a coma as the drugs in her system almost counteracted each other."
The cocktail included:
TWO bottles of Nyquil
TWENTY diet pills, including her favourite brand Clenbuterol.
EIGHTEEN herbal uppers specially ordered over the internet.
EIGHTEEN Piriton antihistamine tablets
TWELVE Vicodin painkillers
TEN sleeping pills
UP TO eight antacid reflux tabs
ONE bottle of stomach upset mixture Pepto Bismol
TEN Zantac tablets, an anti-hangover and indigestion drug.
SIX Ritalin, for her attention deficit disorder issues.
TWO empty bottles of painkiller Oxycontin, known as hillbilly heroin, were also found at her home.
Her pal said: "Britney is always taking something to ease her nerves or keep her weight down.
"But her instability meant she just wanted an instant cure to all her anxieties, so she just kept taking more and more drugs and drink until she felt it worked.
"She told us she made a Purple Monster to calm her nerves. She has had this weird concoction in the past, and only takes it when she is seriously disturbed."
TV deal
FREED Britney Spears is set to reveal all about her week of hell next week on a TV SHOW.
She will bare her soul to celebrity shrink Dr Phil McGraw, a top daytime talk host, after he dashed to her hospital bedside and did a deal yesterday.
My meeting with Britney leaves me convinced more than ever that she is in dire need of both medical and psychological intervention, he said in an exclusive statement to Entertainment Tonight and The Insider.
She records the show tomorrow for screening on Wednesday. News clips are predicted to pull a world audience of more than a BILLION. A pal said: It was all Britneys mums idea.