The New York Post is now reporting that the 34-year-old Birkitt - who was dating Letterman's alleged blackmailer, CBS producer Robert "Joe" Halderman -- revealed in her diary that she continued having flings with the late show host, 62, even after she moved in with Halderman, 51.
Last fall, Letterman and Birkitt hiked together at his lavish Montana ranch, where Letterman and longtime girlfriend Regina Lasko -- the mother of his 5-year-old son, Harry -- wed this past March, the Post reports.
According to the Post, Birkitt insisted to Halderman that she was Letterman's "best friend" and the two had "a platonic relationship."
After reading the contents of Birkitt's diary in December, Halderman discovered she and Letterman were still seeing each other and got angry, the Post reports.
The Post also claims that at least once a week Letterman drove Birkitt to the Norwalk, Conn., home she shared with Halderman since 2004. He even offered her a job as his personal lawyer after she graduated from Benjamin Cardozo Law School.
On his show Monday, Letterman apologized to his wife and poked fun at the scandal. Talking about the cool fall weather, he said, "It's chilly outside my house; chilly inside my house."