FRANK SINATRA JR warns new Rat Pack heir apparent RONAN FARROW  –– that’s my bag, baby!

SINATRA’s family has been rocked by Mia Farrow’s claim that her son Ronan may have been fathered by the music legend.

And no one in the clan is more upset than 69-year-old Frank Sinatra Jr., who’s made a career out of singing his old man’s songs and fears that young, good-looking Ronan could take his place.

“Frank Jr. is extremely angry at Mia for revealing the news that he has a half brother,” said a source. “He doesn’t want to believe it, but he worries it could be true.

“Frank makes his living being his father onstage, singing his songs. He doesn’t want a younger man trying to claim his turf.”

Mia, 68, was married to Ol’ Blue Eyes for 23 months from 1966 to 1968 but claimed in a recent interview that they “never really split up.” When Ronan was born in 1987 (originally named Satchel), Mia was involved with Woody Allen, who was assumed to be the baby’s father. But when asked in the interview if her son could have been fathered by Frank, Mia replied: “Possibly.” Frank would have been 72 when Ronan was born.

The news threw the Sinatra family into chaos. Breaking with her relatives, Nancy Sinatra appar­ently accepted Ronan into the fold with open arms. But the others were outraged, with Frank Sr.’s widow, Barbara, blasting the story as “phony.”

Blue-eyed Ronan, 25, bears a striking resemblance to Frank Sr. but has shown little interest in an entertainment career. A brilliant student, he graduated from Yale Law School at age 21 and is a human rights activist attorney who’s served as a special adviser in the Obama administration. Now, he’s in talks to host a weekend talk show on MSNBC.

But Frank Jr. doesn’t trust Mia’s motives.

“He thinks Mia’s comments are an outrageous attempt to get publicity for her son,” said the source. “Frank Jr. believes Ronan may be launching an entertainment career.

“For years, Frank Jr. has been the only one to keep his dad’s music alive, and he has made a very good living doing it. Now he’s afraid that Ronan will be stepping up to the microphone as another singing son of Frank Sinatra and put his career on the skids.”

Meanwhile, Allen’s representatives called Mia’s interview “fictitious and extravagantly absurd.” But Ronan has made light of the situation, poking fun at Sinatra’s reputation as a skirt-chaser by posting on Twitter: “Listen, we’re all ‘possibly’ Frank Sinatra’s son!”

Author Anthony Summers, who spent five years researching the “My Way” singer’s life for his book “Sinatra: The Life,” says it’s true that Mia and Sinatra shared “a lasting attachment and loyalty to each other.”

As to Mia’s paternity claims, Summers added: “I wouldn’t put it past Frank. It’s very possible.”