Deanna Martin, daughter of Rat Pack crooner Dean Martin, expressed her sadness at Sinatra Jr.'s unexpected passing, telling ET, "I am shocked and devastated at the loss of my lifelong friend Frank Sinatra Jr".

Sinatra Jr was able to provide a link to his father's music after his death, performing his songs and arrangements on tours and especially in Las Vegas.

He worked for years as his father's musical director and conductor, and thrilled sports fans last summer (15) when he performed the U.S. National Anthem at a New York Yankees game.

The younger Sinatra also did some acting, appearing as himself in an episode of The Sopranos and was a reoccurring character on the animated show Family Guy.

Born Francis Wayne Sinatra in 1944, he was the middle child of Frank Sinatra and his first wife, Nancy.

Indeed Sinatra Jr. said he eventually came out from under his father's shadow.

TMZ reports that a spokesperson for the singer reportedly tried to sooth fears by stating Sinatra Jr. was feeling better after admittance to the medical centre.

Sinatra Jr. was married in 1998, but divorced in 2000. He has one son, Michael Sinatra.