MISSOULA, Montana (Reuters) - A federal judge in Montana who acknowledges using official court email to circulate a racist joke about President Barack Obama has apologized and initiated a process to bring a misconduct complaint against himself, court officials said on Thursday.

The government ethics watchdog group Common Cause and the Montana Human Rights Network called for the resignation of the Billings-based judge, Richard Cebull, the chief judge for the U.S. judicial district in Montana, because of his indiscretion.

The email in question came to public light when it was published on Wednesday by the Great Falls Tribune, which received a copy forwarded to the newspaper by someone else in the chain of the email's distribution.

The subject line of the two-paragraph email, sent from Cebull's courthouse email account on February 20 to various friends and acquaintances, according to the newspaper, reads: "A MOM'S MEMORY."

The text says: "Normally I don't send or forward a lot of these, but even by my standards, it was a bit touching. I want all of my friends to feel what I felt when I read this. Hope it touches your heart like it did mine."

The joke that followed included a lewd reference to Obama's biracial parentage.

Cebull was nominated to the federal bench in 2001 by then-President George W. Bush and confirmed by the Senate later that year.

The U.S. Court of Appeals for the Ninth Circuit, which includes Montana, addressed the controversy in a statement.

"Chief District Judge Cebull has publicly acknowledged that he has acted inappropriately," the statement said. "By letter to Chief Judge Alex Kozinski of the Ninth Circuit, Judge Cebull has initiated the process by which a complaint of judicial misconduct will be brought against him."

The Judicial Council of the Ninth Circuit is expected "to act expeditiously in investigating and resolving this matter," it said.

Under the U.S. Constitution, federal judges can normally be removed only through congressional impeachment. That process has rarely resulted in a judge's ouster.

"If he has any respect for his office and for ideals of equality and human dignity on which our country was founded, Judge Cebull will step down today," Bob Edgar, president and CEO of Common Cause, said in a statement on Thursday.

U.S. Senator Jon Tester, a Montana Democrat, said through a spokeswoman that he was "concerned by the situation because it calls into question a lack of judgment by a federal judge."

A representative for Cebull could not be reached for comment on Thursday.

He has publicly admitted sending the email, telling the Great Falls Tribune in an interview published on Wednesday. "I apologize to anybody who is offended by it, and I can obviously understand why people would be offended."

(Reporting by Lori Grannis and Laura Zuckerman; Writing by Alex Dobuzinskis; Editing by Paul Thomasch)