Ellen DeGeneres

Get ready for the image-wrecking tell-all book that Ellen DeGeneres won’t want you to read – a blockbuster that could finally destroy her six-year marriage to Portia de Rossi!

Celebrity journalist Randy Jernigan, a veteran author of no-holds-barred bios, is publishing “The Life and Career of Ellen DeGeneres” in September – and has given National ENQUIRER readers a riveting sneak preview that he claims will rip the lid off the couple’s sordid secrets.

“I think it’s a toxic relationship!” Randy said in an exclusive interview. “Many fans don’t know that Ellen and Portia have had a very tumultuous romance since the very beginning.”

While the talk show queen comes across on air as a friendly, homespun big sister, Randy spoke to many pals and associates of the star and claims his book will expose her private life. Among his startling claims in the tell-all:

Both Ellen and Portia have cheated. Ellen has allegedly cavorted with younger women she met through work, claims Randy, while Portia has had two affairs with former lovers (he won’t name them in the book).

Ellen has battled depression and severe mood swings, and takes out her anger on her wife by becoming extremely controlling, claims Randy.

Portia deals with the drama by drinking and popping pills, said Randy, who claims he confirmed reports she went to rehab in May.

The troubled couple have tried marriage counseling four times – but when they start getting into heavy issues, Ellen can’t handle it and bolts, adds the author.

Fights between the pair are sometimes touched off by their drinking, claims Randy, although Portia is said to have the substance abuse problem.

“I was told that Ellen is often embarrassed by Portia’s drunken antics around friends at dinner parties,” divulged Randy.

“They’ve had blowout fights around their family and friends.”

To make matters worse, Portia is convinced that Ellen, 56, has had crushes on many other women.

Incredibly, sometimes when Ellen likes a woman, she confesses her feelings to Portia, according to Randy. He adds, “Things like that just absolutely drive Portia insane!”

Earlier this year, Portia, 41, “hit the roof” when Ellen was photographed having lunch in West Hollywood with a busty brunette.

“Ellen and the mystery woman, apparently a friend, wore matching T-shirts and were all smiles for photographers,” said an insider.

“It was as if Ellen was sending a message to Portia – letting her know that she wasn’t the only woman in the world.”

But the two have come a long way together. Back in 2004, The ENQUIRER reported how Ellen had been living with photographer Alexandra Hedison, but then met and fell in love with the “Arrested Development” star, who ended her own relationship for Ellen.

“I really do believe Ellen has a deep love for Portia,” said Randy, “and Portia returns that love and affection. The problem is they don’t know how to go about their relationship in an adult way.

“They never learned how to give and take.”

Ellen will undoubtedly be angry when Randy’s new book is published. Explained a source, “She doesn’t want anything to go wrong with the fall launch of her new lifestyle brand, E.D. Hundreds of millions of dollars are riding on it!”