By Jonathan Stempel

WASHINGTON (Reuters) - The Supreme Court on Friday agreed to consider whether a defendant's guilty plea must automatically be thrown out if the presiding federal judge plays a role in plea negotiations.

The case involved the interpretation of federal criminal procedure rules that instruct courts not to participate in such talks, but which say an error can be excused if a defendant's "substantial rights" are not affected.

Anthony Davila, the defendant, had been charged by a Georgia federal grand jury in a 34-count indictment with trying to defraud the government by submitting false income tax refund claims that used the stolen identities of state prison inmates.

Prosecutors said the Internal Revenue Service issued refunds on 87 claims, allowing Davila to bank more than $423,500.

At a February 2010 hearing before a magistrate judge, Davila was denied permission to replace his court-appointed lawyer, who he said never discussed any strategies other than a guilty plea.

The judge told Davila there might not be viable defenses, that a guilty plea could reduce the punishment, and that often "the best advice a lawyer can give" is to plead guilty.

Three months later, Davila pleaded guilty to a conspiracy charge in exchange for dismissal of the other 33 charges. He was later sentenced to about 9-1/2 years in prison.

But in December 2011, the 11th U.S. Circuit Court of Appeals threw out the conviction, ordered that a not guilty plea be entered, and disqualified the magistrate judge.

It said that having a judge take part in plea negotiations, even if motivated by a desire to help a defendant understand his rights, carries an unacceptable risk that a guilty plea may seem coerced and threatens the judge's impartiality. Continued...