Cruel and unusual punishment!

The city has inexplicably banned the New York Post from Brooklyn’s only jail, forcing the roughly 800 inmates there to somehow subsist on a deficient diet of lesser newspapers.

“New York Post Newspaper is no longer accepted,” reads a 5-by-11 sign on Department of Correction letterhead hanging in the visitor’s center of the Brooklyn Detention Complex.

Neither the sign nor DOC officials offered any explanation for the ban.

But The Post has previously been critical of “Brooklyn House,” as the Boerum Hill slammer is commonly known.

An embarrassing Sept. 26, 2018, story revealed that inmates there were using makeshift “fishing poles” to smuggle in contraband from pals down at street level on Atlantic Avenue.

DOC officials did not confirm Saturday whether that story prompted its Post ban.

“We are looking into this — this sign will be removed,” DOC spokesman Peter Thorne said Saturday.

But the outrageous ban has already sparked outrage — even from lefty liberals who sometimes butt heads with the city’s greatest paper.

“I’ve never come across the wholesale banning of a non-pornographic, mainstream newspaper before,” said civil rights lawyer Ron Kuby.

“It’s unheard of, it’s unconstitutional, it’s unwise and it makes people look ­really silly,” he added.

Jails can ban reading material that, for instance, describes how to escape or construct weapons.

A single day’s newspaper can also be banned if it creates a safety risk by revealing nonpublic information about an investigation, Kuby noted — but not an entire publication.

Civil rights lawyer Norman Siegel agreed. “Freedom of the press is a fundamental right in America,” he said.

“I can’t think of any reasons to prohibit the New York Post,” he added. “Even though there’s moments — more than moments, maybe days — when I disagree with what’s in the New York Post.”

Additional reporting by Kenneth Garger, Kevin ­Fasick and Laura Italiano