In his ruling, Judge Stephen Robinson states that Breen, a former federal prosecutor, "is an actual witness to the alleged crime."
"There are a host of scenarios in which, even if Mr. Breen's name were not mentioned during testimony, the conflict would still not be addressed," Robinson said. "For example, Mr. Breen may be constrained from making certain arguments, or may be tempted to minimize his conduct at the expense of his client, and thus could not be said to advocate effectively for the Defendant."
Robinson gave Kerik 30 days to find a new lawyer.
Breen appeared with Kerik last year he was indicted earlier this month on 16 charges including accusations of lying to the White House, filing false income taxes, tampering with witnesses, and avoiding the nanny tax.
Kerik pleaded not guilty.
Prosecutors intend to call a former Kerik attorney, Joseph Tacopina, as a witness to testify about false claims made to the Bronx district attorney's office during a previous investigation.
Besides questioning Tacopina, federal prosecutors may subpoena Breen, or "he would be an unsworn witness ... to facts that go to the heart of many of the charges in the indictment."
On another matter -- a reportedly false loan application -- "the defendant's admissions to Tacopina and Breen are tantamount to a confession," prosecutors have said.
Breen's former law firm is suing Kerik over Breen's legal fees in the Bronx matter, which "further complicates Breen's position," according to federal prosecutors.
As New York's mayor, Giuliani appointed Kerik police commissioner in 2000, and Giuliani later endorsed Kerik's nomination to head the federal Department of Homeland Security.
Days after President Bush introduced Kerik as his nominee, however, Kerik announced he was withdrawing his name because of tax issues involving his former nanny.
Giuliani has said he should have done a better job of vetting Kerik.
