In the last months of her life, WHITNEY HOUSTON received regu­lar deliveries from a drug dealer – who also secretly supplied drugs and booze to her teenage daughter BOBBI KRISTINA!

And in a shocking new development, The ENQUIRER has learned that Whitney – who’d bounced in and out of rehab – spent much of her last six months locked in her bedroom getting high.

“A drug dealer named Jay came to Whitney’s house in the evenings,” revealed a source close to the beautiful diva, who died at age 48 on Feb. 11.

“Jay’s a black guy in his early 40s who always wore jeans, a T-shirt, sneakers, a jacket that matched his jeans and dark sunglasses.”

Jay was familiar with the family, according to the in­sider, and drove a black Cadillac Escalade with tinted windows to Whitney’s Atlanta-area townhouse in a gated community, where one of her companions paid him in cash at the front door.

“Whitney would hand a wad of cash to the third party – a trusted friend. It contained hundreds of dol­lars, and the pal would give Jay the money,” the source revealed.

After he got paid, Jay would hand over a brown paper bag “with the top folded down,” the source said.

“No one except Whitney ever saw what was inside the bag,” the source add­ed. “The friend would hand the bag to Whitney, who was waiting for it behind her bedroom door.

“Whitney would always say ‘thank you,’ and then close her bedroom door quickly.”

In a sickening twist, the insider reveals Jay was not only selling drugs to Whit­ney, he was also supplying drugs and booze to her 18-year-old daughter, but the Grammy-winning singer never knew it.

“Bobbi Kristina bought drugs from Jay only when Whitney was out of town,” the source revealed.

 “She’s underage and can’t buy alcohol, so Jay would also bring her large bottles of vodka. She’d take the booze to her bedroom and drink it there.”

Bobbi Kristina also threw wild parties when her mother was away, inviting 50 to 60 friends over about twice a week, said the source.

“Bobbi and her friends would tear up the house, drinking, dancing, screaming and doing drugs until they passed out,” said the insider. “It was pretty out of control.”

But when Whitney came back home, she didn’t seem to care.

“Whitney would say, ‘She’s young! She’s having fun!’ and leave it at that,” the source recalled.

For ALL the details and EXCLUSIVE PICTURES – it’s the ENQUIRER – still on sale!