By Jane Sutton

MIAMI, May 16 (Reuters) - Charges against five Guantanamo prisoners accused of plotting the Sept. 11 attacks should be thrown out because they were improperly influenced by a Pentagon legal adviser, U.S. military lawyers said in documents filed on Friday.

Also on Friday, a U.S. military judge postponed the Guantanamo trial of Osama bin Laden's driver, Salim Hamdan, until July 21, to allow time to assess his mental competency.

Hamdan was to be the first prisoner tried in the U.S. war crimes court at the Guantanamo naval base in Cuba.

The Guantanamo tribunals are the first U.S. war crimes tribunals since World War Two and have faced steady criticism from human rights activists and reversals in U.S. courts.

The tribunals were established after Sept. 11, 2001 to try non-American captives whom the Bush administration considers "enemy combatants," who are not entitled to the legal protections granted to soldiers and civilians.

In the case of alleged Sept. 11 mastermind Khalid Sheikh Mohammed and four other prisoners who could face execution if convicted, the military defense lawyers said the charges were tainted by meddling and "overreaching" on the part of Air Force Brig. Gen. Thomas Hartmann.

Hartmann was assigned to provide impartial legal advice to the Pentagon appointee overseeing the Guantanamo trials.

But the former chief prosecutor of the tribunals testified last month that Hartmann essentially took over the prosecution team, pushing it to use evidence obtained through coercion and demanding "sexy" cases that would pique the interest of the American public.

Military defense lawyers asked the tribunals' chief judge, Marine Col. Ralph Kohlmann, to dismiss the charges on grounds that Hartmann was so heavily involved in drafting them that he "failed to retain the required independence."

FACING DEATH PENALTY

"When the government seeks the death penalty, it must not reduce legal procedures to mere formalities. The integrity of the system must be upheld, regardless of the nature of the crimes charged or identity of the accused," they said in the documents. "The accused are entitled to due process and a fair trial"

Mohammed and the other four -- Ali Abdul Aziz Ali, Ramzi Binalshibh, Mustafa Ahmed al-Hawsawi and Walid bin Attash -- were tentatively scheduled for arraignment at Guantanamo on June 5.

They are accused of conspiring with al Qaeda to murder civilians and with 2,973 counts of murder, one for each person killed when hijacked passenger planes crashed into the World Trade Center, the Pentagon and a Pennsylvania field on Sept. 11.

The judge in the Hamdan case had already barred Hartmann from further involvement in those proceedings.

Hamdan was scheduled to go to trial in early June but a military judge, Navy Capt. Keith Allred, delayed it until July 21. He ordered that a mental competency review be completed by June 13, for Hamdan, a Yemeni who was to be the first captive tried in the Guantanamo war court.

His lawyers said that more than six years of detention and harsh treatment at Guantanamo had left Hamdan mentally impaired and compromised his ability to assist in his defense.

Hamdan told the judge last month he would boycott his trial because he had no hope of receiving fair treatment. He would face life in prison if convicted of conspiring with al Qaeda and providing material support for terrorism.

Hamdan was captured in Afghanistan in November 2001, shortly after U.S.-led forces invaded to oust the Taliban and their al Qaeda cohorts. He has said he never joined al Qaeda but took a job in bin Laden's motor pool because he needed the $200 monthly salary.

Prosecutors say he was a trusted al Qaeda insider who transported weapons for the group and helped bin Laden escape U.S. forces in Afghanistan. (Editing by Michael Christie)

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