The judge in the Iran-Contra case dismissed major charges against Oliver North on Friday, saying "the court is powerless" to do otherwise. White House lawyers, meanwhile, moved to stop North's effort to force President Reagan and President-elect George Bush to testify.

In trying to keep Reagan and Bush from being called as witnesses by North, the administration raised the threat of a constitutional battle "of potentially major dimensions" and said it has been offering to respond in writing since last Sept. 28 to any "reasonable written questions" the former White House aide might ask as part of his defense.Twelve criminal charges remain against North, and the issue of dealing with classified information in open court continues to be a problem. Attorneys for both sides met privately in the judge's chambers after Friday's open session to discuss matters relating to the Classified Information Procedures Act.

North's lawyers have subpoenaed Reagan and Bush to testify at the trial, now set for Jan. 31, but no sitting president has ever testified in person in court.

Disclosure of White House willingness to answer questions in writing came in a 27-page brief by Attorney General Dick Thornburgh, who said that North "has not identified any of the issues on which he wishes to examine the president or president-elect and has not demonstrated the need for their testimony."

"Until he does so," wrote Thornburgh, "there can be no justification for precipitating constitutional litigation of potentially major dimensions by compelling an in-court appearance by a sitting or former president."

He said Reagan "will consider furnishing appropriate information in writing in response to reasonable written questions, if limited to material subject matter." He said Bush "joins that offer."

North's lawyers, meanwhile, served the Justice Department with subpoenas for Reagan and Bush to turn over various personal papers for use by the defense, said department spokesman Loye Miller.

The administration filed its motion to quash the subpoenas for Reagan's and Bush's personal testimony four hours after U.S. District Judge Gerhard Gesell declared at a hearing that he had no power to stop the government's abandonment of conspiracy and theft counts against North.

Because of national security concerns, Thornburgh and independent counsel Lawrence Walsh apparently "are unable to prosecute" North on charges of diverting more than $14 million in U.S.-Iran arms sale proceeds to the Nicaraguan rebels, said Gesell.

"The court is powerless to direct a case to proceed," the judge said.

Walsh asked on Jan. 5 that Gesell dismiss the charges of conspiracy and theft of government property, citing North's insistence on introducing classified information in his defense and the refusal by the Reagan administration to release material the judge ruled North must be able to use at trial.

Gesell refused to dismiss the charges until he received an affidavit from Thornburgh outlining the administration's decision to withhold classified data.

Thornburgh stopped short of that, supplying instead a two-page declaration saying that if Walsh had disagreed with the Reagan administration's refusal to declassify material, the attorney general would have filed an affidavit "objecting to disclosure of this classified information." Gesell said Thornburgh's filing met the "minimum standard" under the Classified Information Procedures Act.

The classified material in question includes names of Central American countries and verbatim texts of intelligence reports.

Meanwhile, John Keker, the head of Walsh's prosecution team, said North's defense team is subpoenaing 50 CIA agents in the case, some of whom are now serving overseas and are concerned about having to appear on 48 hours notice at the request of the defense once a trial gets under way.