(Adds court ruling rejecting immunity for Karadzic)

By Reed Stevenson

THE HAGUE, Dec 18 (Reuters) - The UN war crimes tribunal for former Yugoslavia hopes wanted Bosnian Serb Ratko Mladic can be caught in time to put him on trial with his old political leader Radovan Karadzic, the chief prosecutor said on Thursday.

The tribunal faces an informal deadline to wrap up proceedings by 2012, rising costs, staff departures and two fugitives still on the run, while preparing for the trial next year of its top-ranking detainee, Karadzic, for genocide.

If Mladic, Bosnian Serb military chief during the 1992-95 war, were captured before the start of Karadzic's trial, the case against him could be "swiftly joined" and they could be tried more efficiently together, prosecutor Serge Brammertz told reporters in The Hague.

"This is the only good solution," he said. Both suspects are accused of genocide.

Separately, in documents made available on Thursday, tribunal judges rejected claims by Karadzic that he was immune from prosecution because of a secret immunity deal with former U.S. peace envoy Richard Holbrooke in a meeting in Belgrade in mid-July 1996.

Any immunity agreement "would be invalid under international law," judges said, but also ordered the prosecution to hand over any documents it may have concerning an agreement at that time. Holbrooke has repeatedly denied making any secret immunity deal.

Brammertz said in a report to the U.N. Security Council last week Serbia's cooperation with the tribunal had improved but more work was needed to ensure the arrest of Mladic and Goran Hadzic, a former Serbian politician also wanted for war crimes.

Mladic is believed to be in Serbia, protected by nationalist hardliners.

Brammertz said combining his case with Karadzic's would probably take a matter of days. "We are aware of the need to be as efficient as possible," he said.

Arrested in July in Belgrade, where he had assumed a new identity and worked as an alternative healer, Karadzic faces 11 charges of war crimes and crimes against humanity, including genocide over the 1995 Srebrenica massacre of 8,000 Muslims.

Created in 1993, the International Criminal Tribunal for the former Yugoslavia has said that it expects to wrap up its cases by the end of 2010 and appeals by end-2011. The chief prosecutor admitted however that "work could easily go into 2012".

"It is impossible to imagine that the Security Council will not allow us to do the job we are meant to do" and shut the tribunal with trials incomplete, Brammertz said.

"It would be a legal catastrophe," he added.

The tribunal's highest-profile suspect, former Serbian President Slobodan Milosevic, died in 2006 during his marathon war crimes trial. (Editing by Jon Boyle)

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