A touching letter from notorious gangster Al Capone to his son from prison is up for sale.

The three-page "intimate" letter - which is being auctioned off next week -. is addressed to Capone's son, Albert "Sonny" Capone.

The mobster signed it, "Love & Kisses, Your Dear Dad Alphonse Capone #85," which was his number at the Alcatraz prison in San Francisco Bay.

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"Junior keep up the way you are doing, and don't let nothing get you down. When you get the blues, Sonny, put on one of the records with songs I wrote you about," Capone wrote.

Later, he added: "Well heart of mine, sure hope things come our way for next year, then I'll be there in your arms."

Robert Livingston, executive vice president of Boston-based RR Auction, which is handling Monday's auction at a hotel in Cambridge, Massachusetts, said: "It's an exceedingly rare personal letter showing the softer side of the notorious gangster."

The letter is expected to fetch around 50,000 dollars (£38,000).

"You'd think that figures like this would be despised, but instead, they're kind of deified in the consciousness of the American public," Mr Livingston added.

The legendary Brooklyn-born mobster, who ruled gangland Chicago during Prohibition, is best known for his 1929 "Valentine's Day Massacre" of seven members of rival bootlegger Bugs Moran's gang.

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In 1931 Capone was charged with income tax evasion. He was eventually convicted and sentenced to 11 years in prison, much of which he spent at Alcatraz. Capone was finally released a few years early in 1938 and returned to his Miami Beach mansion. Riddled with syphilis, he suffered a stroke and died in 1947 at age 48.

The letter to his then college-aged son is likely to have been written in 1938, four years after the man dubbed "Public Enemy No. 1" was transferred to Alcatraz.

In a somewhat surprisingly cheerful tone, his letter describes the daily grind in prison, which Capone tried to relieve by playing banjo and mandola. While at Alcatraz, Capone put together an inmate band he dubbed The Rock Islanders.

"Sonny I got a Song like Rainbow on the River, that was sung by Bobby Breen, in the Rainbow on the River picture, I sure hope you seen it as we saw it out here," he wrote. "... When I come home, I will play not only that song, but about 500 more, and all mostly Theme Songs from the best Shows. In other words Junior, there isn't a Song written that I can't play."

Capone ended the letter encouraging his son to stay strong.

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"Well Sonny keep up your chin, and don't worry about your dear Dad, and when again you allowed a vacation, I want you and your dear Mother to come here together, as I sure would love to see you," he wrote.