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An atheist organization is blasting the U.S. Postal Service for its plan to honor Mother Teresa with a commemorative stamp, saying it violates postal regulations against honoring "individuals whose principal achievements are associated with religious undertakings."

The Freedom from Religion Foundation is urging its supporters to boycott the stamp — and also to engage in a letter-writing campaign to spread the word about what it calls the "darker side" of Mother Teresa.

The stamp — set to be released on Aug. 26, which would have been Mother Teresa's 100th birthday — will recognize the 1979 Nobel Peace Prize winner for her humanitarian work, the Postal Service announced last month.

"Noted for her compassion toward the poor and suffering, Mother Teresa, a diminutive Roman Catholic nun and honorary U.S. citizen, served the sick and destitute of India and the world for nearly 50 years," the Postal Service said in a press release. "Her humility and compassion, as well as her respect for the innate worth and dignity of humankind, inspired people of all ages and backgrounds to work on behalf of the world’s poorest populations."

But Freedom from Religion Foundation spokeswoman Annie Laurie Gaylor says issuing the stamp runs against Postal Service regulations.

"Mother Teresa is principally known as a religious figure who ran a religious institution. You can't really separate her being a nun and being a Roman Catholic from everything she did," Gaylor told FoxNews.com.

Postal Service spokesman Roy Betts expressed surprise at the protest, given the long list of previous honorees with strong religious backgrounds, including Malcolm X, the former chief spokesman for the Nation of Islam, and the Rev. Dr. Martin Luther King Jr., a Baptist minister and co-founder of the Southern Christian Leadership Conference.

"In fact we honored Father Flanagan in 1986 for his humanitarian work. This has nothing to do with religion or faith," Betts told FoxNews.com.

Click here to see other controversial U.S. stamps.

Gaylor said the atheist group opposed Father Flanagan's stamp but not those for King and Malcolm X, because she said they were known for their civil rights activities, not for their religion.

Martin Luther King "just happened to be a minister," and "Malcolm X was not principally known for being a religious figure," she said.

"And he's not called Father Malcolm X like Mother Teresa. I mean, even her name is a Roman Catholic honorific."

Gaylor said Mother Teresa infused Catholicism into her secular honors — including an "anti-abortion rant" during her Nobel Prize acceptance speech — and that even her humanitarian work was controversial.

"There was criticism by the end of her life that she turned what was a tiny charity into an extremely wealthy charity that had the means to provide better care than it did," Gaylor said. "...There's this knee jerk response that everything she did was humanitarian, and I think many people would differ that what she was doing was to promote religion, and what she wanted to do was baptize people before they die, and that doesn't have a secular purpose for a stamp."

But the Postal Service said the commemorative stamp has nothing to do with Mother Teresa's religion.

"Mother Teresa is not being honored because of her religion, she's being honored for her work with the poor and her acts of humanitarian relief," Betts told FoxNews.com.

"Her contribution to the world as a humanitarian speaks for itself and is unprecedented," he added.

Some atheists, too, spoke out against the group's objections, including Bruce Sheiman, author of "An Atheist Defends Religion." He said the Freedom from Religion Foundation is being "hypocritical" and really "stepping over the line."

"Clearly there are a number of things that you can point to and say it's religious and a number of things you can point to and say that it's areligious," Sheiman told FoxNews.com. "So it really doesn't make sense to protest it."

He said the Foundation's campaign stems from concern that the abundance of humanitarian work done by believers will overshadow that done by atheists.

"Like billboards and bus ads, this is just part of the whole campaign that they're doing to make non-belief more visible," he said.

Gaylor said the foundation's only concern is the "other things that deserve to be commemorated but are not because the people behind it didn't have the power of the Catholic church."

"It's enormously difficult to get them," she said, referring to commemorative stamps, "and people have huge campaigns, and to me this speaks of the power of the Roman Catholic Church in hierarchy.

"They want to make her a saint and this is part of the PR machine."

The Foundation is encouraging its supporters to purchase the new stamp honoring the late actress Katharine Hepburn, who was an atheist, instead — or any of the other 2010 stamps, which include cartoonist Bill Mauldin, singer Kate Smith, filmmaker Oscar Micheaux, painter Winslow Homer and poet Julia de Burgos.

Betts said that despite the Foundation's accusations and letter-writing campaign, "The response to Mother Teresa has been overwhelmingly in favor of this stamp."

He said the Mother Teresa stamp, like other stamp subjects, will "stand the test of time, reflect the cultural diversity of our nation and have broad national appeal."