A federal judge has rejected, for now, the effort by a former ambassador, Joseph Wilson IV, to escape testifying in the trial of Vice President Cheney’s former chief of staff, I. Lewis Libby Jr.

“Since Ambassador Wilson has yet to be actually called as a witness, he has not been inconvenienced, nor can he establish at this time that the subpoena is unreasonable or oppressive. Accordingly, his motion is premature,” Judge Reggie Walton wrote in an order dated Tuesday.

Last month, Mr. Wilson’s lawyers moved to block his testimony after he was served with a subpoena by Mr. Libby’s defense team. The former diplomat is the husband of a veteran CIA officer, Valerie Plame, whose named appeared in a syndicated newspaper column in 2003, triggering the investigation that led to Mr. Libby’s indictment.

Judge Walton said it was “impossible” to evaluate the relevancy of Mr. Wilson’s testimony until the trial, which is scheduled to begin next week, gets underway. The judge said Mr. Wilson could renew his motion to quash the subpoena if the defense actually attempts to call him as a witness. The defense has said that it is not certain whether it will seek to call Mr. Wilson and has offered to give him 48 hours notice if he is called.

Mr. Wilson’s legal team claimed that the subpoena was an improper effort to undermine a separate civil suit the former diplomat and his wife have brought, charging Mr. Libby and other senior Bush Administration officials with invading the couple’s privacy, harming their work prospects, and violating their constitutional rights by conspiring to expose her wife’s classified employment with the CIA.

No criminal charge has been brought over the apparent leaks, but Mr. Libby was charged with obstruction of justice and perjury for allegedly giving false accounts of his involvement to the FBI and a grand jury. He has pleaded not guilty and blamed memory failures for any misstatements.