TV legend Johnny Carson was so convinced his second wife was bedding football great Frank Gifford, he mounted an undercover mission to catch her cheating!

That’s the shocking revelation from Johnny’s former attorney and longtime confidant Henry Bushkin – who also claims the popular Tonight Show host cracked jokes about the New York Giant-turned-broadcaster to hide his grief and went on a drinking binge after finding the evidence he believed pointed to an affair.

"Why Frank Gifford?" the comic asked, according to a book proposal by Bushkin. "What’s that (bleep) got that I don’t have? That guy plays three positions on the field. I could never get Joanne to go for more than two!"

Joanne, whose nine-year marriage to Johnny ended in 1972, has dismissed the story as "a nice piece of fiction."

But Bushkin says Johnny was so convinced of his wife’s cheating that he cooked up a scheme to spy on her.

Not long after Bushkin met Johnny in 1970, the TV host asked him and a private detective to go to an apartment Joanne kept in New York, he writes. After tipping the building manager $100 to gain entry to the residence, he claims they found framed photos of Gifford and what appeared to be his clothes in her closet.

Johnny filed for divorce the following day, and Bushkin says he received a 2 a.m. phone call a few days later from a distraught, "obviously tanked" Carson.

Joanne, now 78, admits to knowing Gifford, saying they had "a very special friendship when I was about 21 and single," but insists she had no romance with him during her marriage to Johnny.