WASHINGTON (CNN) -- A federal judge used tough language to block efforts by the Obama administration to limit the legal rights of terror suspects held at the Guantanamo Bay military prison in Cuba, ruling Thursday that proposed changes were an "illegitimate exercise of executive power."

Officials of the departments of Justice and Defense had claimed they alone should decide when the prisoners deserve regular access to their attorneys.

But in a 32-page ruling, Judge Royce Lamberth said federal courts had proper authority to decide the matter, and criticized the executive branch for recently changing the procedures, when he said the current system was working well.

"The old maxim 'if it ain't broke, don't fix it' would seem to caution against altering a counsel-access regime that has proven safe, efficient, and eminently workable," said Lamberth. "Indeed," he added, "the government had no answer when the court posed this question in oral arguments" last month.

"Access to the courts means nothing without access to counsel," added the judge.

Justice Department lawyers said they have started restricting when Guantanamo prisoners could challenge their detention in the Washington-based federal court. If approved, any relaxing of the rules would be made on a case-by-case basis at the exclusive discretion of military officials, not by the courts.

At issue is whether a Supreme Court decision on detainee rights from 2008 gives federal courts the ultimate power to control so-called "habeas" petitions from foreign combatants in U.S. military custody. Volunteer private lawyers say they deserve regular access to their imprisoned clients, even if there is no active habeas challenge pending in court, or any pending charges.

Under the proposed changes, the Navy base commander at Guantanamo would have sole veto power over attorney access, as well as access to classified material, including information provided directly by the detainees from interrogations.

"The dispute thus before the court, though important, is quite narrow," said the government in its earlier legal filing. "The only question presented is whether detainees who have neither current nor impending habeas petitions are entitled to" challenge continued access to counsel. "The answer to that question is 'no.'"

Lamberth's Washington-based federal court has been handling the many appeals filed by the prisoners. There are currently 168 detainees -- all male -- in the Guantanamo facility, most of whom do not have pending charges. Five Muslim men labeled "high-value detainees" are being prosecuted before a military commission for their alleged leadership roles in the 9/11 terrorist attacks.

In the so-called Boumediene ruling in 2008, the high court said "enemy combatants" held overseas in U.S. military custody have a right to a "meaningful review" of their detention in the civilian legal justice system. It would force the government to present evidence and justify keeping the prisoners indefinitely, without charges. But a federal appeals court in Washington has since refused to order the release of any detainee filing a habeas corpus writ, in some cases rejecting such orders from lower-court judges.

Civil rights groups applauded the court opinion.

"The court has correctly recognized the government's attempt to restrict attorney access to the men at Guantanamo as the latest in a ten year history of successive efforts to "delay, hinder, or prevent access to the courts," said Vincent Warren, executive director of the Center for Constitutional Rights, which has been representing many of the Gitmo prisoners. "The new rules came out of the blue and can only be seen as an effort to punish the men at Guantanamo for exercising their right to challenge their detention. These rules would have given the government unfettered control over Guantanamo. As the court said, the executive cannot be trusted with such power."

The administration has argued it does not seek to restrict lawyers who have an active legal appeal, but that the rights of detainees shrink once they have filed their first habeas challenge. The military says lawyers must now agree to the new conditions in order to have continued access to their clients and to any classified information the military would deem to release.

And lawyers would be prohibited from using any information they gather that might help the prisoners appearing before a Periodic Review Board. Review boards are newly designed panels of military officials to decide whether a Guantanamo inmate should continue to be held, and whether that person is a national security threat.

Those boards were put in place by President Barack Obama by executive order, but have not been fully implemented.

"Executive Order 13,567 does not provide detainees who undergo PRB review with a judicially enforceable right to counsel, or any justification for asking the Court to impose a counsel-access regime on the PRB process other than the one developed, per the Order's direction, by the Secretary of Defense," said the government. "As a general matter, executive orders are viewed as management tools for implementing the President's policies, not as legally binding documents that may be enforced against the Executive Branch."

The government said the court's power to intervene was limited, and had urged Judge Lamberth to deny the request guaranteeing attorney access. But in strong language the judge refused.

"The court has an obligation to assure that those seeking to challenge their executive detention by petitioning for habeas relief have adequate, effective and meaningful access to the courts," said Lamberth. "And it is undisputed that petitioners here have a continuing right to seek habeas relief. It follows that petitioners have an ongoing right to access the courts and, necessarily, to consult with counsel. Therefore, the Government's attempt to supersede the court's authority is an illegitimate exercise of executive power.

"The court, whose duty it is to secure an individual's liberty from unauthorized and illegal Executive confinement, cannot now tell a prisoner that he must beg leave of the executive's grace before the court will involve itself. This very notion offends the separation-of-powers principles and our constitutional scheme."

The Justice Department now has the option of asking a federal appeals court to intervene.

The case is In re: Guantanamo Bay Detainee Continued Access to Counsel (1:04-cv-1254).