(Adds charges dismissed against five detainees, background)

By Andrew Gray

WASHINGTON, Oct 21 (Reuters) - The Guantanamo Bay prison for terrorism suspects will not close under President George W. Bush but the next president and Congress should move swiftly to shut it down, Defense Secretary Robert Gates said on Tuesday.

Gates, who has advocated closing the controversial prison at a U.S. naval base in Cuba since taking over at the Pentagon in late 2006, called Guantanamo "a real liability for the United States."

In comments to news agency reporters at the Pentagon, Gates said the Bush administration had concluded that closing Guantanamo would require legislation but there was little chance of getting it passed in a heated political climate with the presidential and congressional election on Nov. 4.

"This is an issue that'll have to be addressed early on by a new administration," Gates said.

The prison, and the justice system for its detainees, has been widely condemned by human rights groups and governments around the world, including close allies of the United States, who say it does not meet international legal standards.

Asked to state whether the prison would be closed before the Bush administration leaves office on Jan. 20, Gates replied: "No, regretfully."

Both candidates seeking to replace Bush in the presidential election -- Republican John McCain and Democrat Barack Obama -- have pledged to close the detention center.

Gates said that would require legislation, for example to ensure that no Guantanamo detainees would have the right to emigrate to the United States.

LATEST SETBACK

In the latest setback to Guantanamo prosecutions, a Pentagon official dismissed all charges against five detainees on Tuesday, including a British resident who says he falsely confessed to a radioactive "dirty bomb" plot under torture.

About 255 suspected members of al Qaeda, the Taliban and other associated groups are imprisoned at Guantanamo, which once held as many as 600 detainees.

The Pentagon plans to try about 80 of them for war crimes but so far has managed only one complete trial and one guilty plea to avoid a trial. Officials have not yet come up with a plan to deal with the other detainees.

Rights groups have also criticized interrogation techniques at Guantanamo but Gates appeared to suggest the prison's reputation dated from its early days, when images of detainees in the open air behind barbed wire shocked many people.

"It's probably one of the best-run prisons in the world today but the reality is, because of the past, it is a real liability for the United States," he said.

"My belief is the new administration and the new Congress ought to address this issue so we can get past this issue and close it."

Gates' position on Guantanamo -- in line with the presidential candidates and at odds with more hawkish members of the Bush administration -- has stoked speculation he could stay on in the next administration.

He has tried to play down that speculation while not completely dismissing it. Gates said on Tuesday he still planned to leave government when this administration leaves office. (Editing by Kristin Roberts and John O'Callaghan)

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