At stake are millions of dollars in support money and custody of their two boys, Sean Preson, 22 months, and Jayden James, 10 months.

Kevin hired his private investigator last March- as soon as alcohol-abusing Britney came out of Promises Treatment Center in Malibu, revealed a source.

Among devastating charges, Kevin claims his spy has caught a booze-swilling Britney sleeping with a rehab buddy and her drug counselor.

And Britney claims her husband has turned his home into an orgy pit- even when their two youngsters are there.

The singer recently made a last-ditch effort to put her shattered family back together- and desperately tried to reconcile with Kevin. But he resisted all her Humpty-Dumpty repair efforts.

Then several weeks ago, they got into a furious fight over the phone.

He claimed she was drunk- and she tore into him, said the source.

“Through tears, she told him she knew he was having parties at the house while he had the children and sleeping with his ex-flame Shar Jackson.�?

Here- exclusively in The ENQUIRER- is the first sources say the Fighting Federlines claim to have on each other:

Kevin is parting with wild women and drug-using pals.

“Britney was informed that Kevin has had several pals visit his home while he has the boys- and his friends are welcome to drink, smoke marijuana and party with half-naked girls,�? disclosed the source. “And he has had several girls over for one night stands. The boys don’t ever come out of their rooms, and no one is allowed back in their rooms.�?

Kevin has been getting close with Shar again.

“Britney is convinced that Kevin and Shar never really broke up and that all along Kevin planned to stay with Britney for a while, take her to the cleaners in divorce court- and then go running back to Shar,�? said the source.

Britney’s investigator has identified times he says Kevin has been with Shar, the mother of two of his children.

“Britney knows that Shar has spent the night several times with the boys at Kevin’s. They all play house, and Shar brings all four of her kids,�? said the source.