INSIDE the carnal world of David Letterman’s lover Stephanie and her secret war with his wife Regina for dominance of the gap toothed funnyman’s love!

A love struck David Letterman showered his young mistress with millions of dollars, five-star vacations and coveted promotions during their tawdry secret affair, The ENQUIRER has learned exclusively.

The shameless 62-year-old talk-show host put his girlfriend in charge of his Indy race car team, brazenly took her on trips with his family, and even had her re-enact his favorite "Stupid Pet Tricks" – with her as the pet, sources say.

That’s the shocking inside story of Letterman’s illicit romance with his 34-year-old personal assistant Stephanie Birkitt – an affair that continued after he married longtime girlfriend Regina Lasko last March.

Insiders say Stephanie has competed with Regina – mother of Letterman’s 5-year-old son Harry – for the Late Show host’s affections since she started working for him in 1996.

"It was like Stephanie was in a secret war with Regina that Regina knew nothing about. Stephanie once boasted, ‘Dave may go home to her, but I’m the ONE he fantasizes about!’" an insider told The ENQUIRER.

For the full story of Stephanie Birkitt‘s steamy love affair with the Late Show host and to see exclusive PIX of their top-secret love nest pick up the new ENQUIRER — on sale NOW!