Frank Sinatra, a popular crooner, had called Elvis Presley’s music “degenerate and phony, sung by cretinous goons.” However, when Elvis became the biggest star in the world, Sinatra had no choice but to have him on his show.

Despite everything that was going on behind the scenes, the two men were all smiles and even performed medleys of each other’s songs.

During the show, Sinatra even danced with Elvis, even though, just two years earlier, he had said of rock ‘n roll: “It is the most brutal, ugly, degenerate, vicious form of expression it has been my displeasure to hear…

“It fosters almost totally negative and destructive reactions in young people.

“It smells phony and false.”

Behind the scenes, Elvis was having a passionate s^xual affair with Juliet Prowse, who was Sinatra’s co-star and girlfriend.

Prowse had been with Sinatra since they met the previous year on the set of the musical Can-Can.

However, in April 1960, Prowse started filming GI Blues with Elvis, playing nightclub dancer Lili.

As so often happened, Elvis soon started an affair with his beautiful co-star.

Extraordinarily, Prowse openly declared at the time that Sinatra knew about it.

Prowse said, “Frank and I are mature people.

We don’t go for this teenage bit about going steady and all that jazz.” In fact, their relationship only intensified after her fling with Elvis.

She would also join Sinatra when he had engagements in Las Vegas, and he proposed to her in 1962.

However, it soon burned out, with Prowse reportedly wanting to focus on her own career.

She later admitted: “I was as much flattered as I was in love.

“He was a complex person, and after a few drinks he could be very difficult.”

Sinatra, meanwhile, became friends with Elvis, as their own careers started to overlap more and more in the late 1960s and early 1970s with both headlining lucrative residencies in Las Vegas.

He even made a desperate call to The King begging him to change his life.

Both stars also became friends with Tom Jones, when they all headlined their own shows in the casino mecca.

Years later, Jones’ publicity man, friend, and journalist Chris Hitchens revealed a candid conversation where Sinatra confessed he had tried to save The King.

The King had been admitted to hospital in August 1975, with liver problems exacerbated by his heavy drug use.

Friends and family were starting to worry about him, which prompted an extraordinary intervention from his fellow star.

Hitchens described Sinatra telling them: “When I called the hospital in Memphis, the girl on the switchboard asked: Who’s calling?’ and when I replied: ‘Frank Sinatra”’I fully expected her to say: ‘Oh, yeah, and I’m the Queen of England’ or some such dumb line.

But she must have recognised my voice because, a few seconds later, Elvis came on the line.

“I told him he’s got to look after himself and quit fooling around.

“He’s too young to die, and I told him so.”

Unfortunately, Elvis would die exactly two years later on August 16, 1977.

His cause of death was attributed to heart failure, exacerbated by systemic and prolonged overuse of prescription drugs.

He was, indeed, far too young at just 42.