By Thomas Escritt and Fatos Bytyci

AMSTERDAM/PRISTINA (Reuters) - Ramush Haradinaj, a former guerrilla fighter in Kosovo who served briefly as prime minister, was acquitted of war crimes for a second time on Thursday, clearing the way for his return to mainstream politics but angering Serbia.

The retrial verdict by a United Nations court in The Hague comes on the heels of the acquittal on appeal two weeks ago of top Croatian general Ante Gotovina, fuelling nationalist accusations in Serbia that the court is biased against them.

The verdict, and Haradinaj's return to frontline campaigning, could undermine a new effort by the European Union to encourage Serbia and Kosovo to mend ties almost five years after the former southern Serbian province declared independence with the backing of the West.

Haradinaj was accused with two accomplices of persecuting ethnic Serbs in an effort to drive them out of Kosovo, during a 1998-99 war between guerrillas of the Kosovo Liberation Army (KLA) and security forces under late Serbian strongman Slobodan Milosevic.

Judges ruled there was no evidence to support the charges of crimes against humanity.

"On the contrary, the evidence establishes that, when he heard about the mistreatment of individuals, Haradinaj said no such thing should happen because this is damaging of our cause," the presiding judge, Bakone Justice Moloto, said.

There were gasps and cheers in the courtroom, and fireworks in Kosovo's capital, Pristina.

Haradinaj was prime minister for several months in 2005 but resigned when he was first charged. He was acquitted in 2008 but appeal judges ordered a partial retrial, saying the prosecution should have been given more time to make its case.

Prosecutors have long been dogged by problems of witness intimidation in Kosovo, where the KLA is still revered and clan loyalties run deep. Kosovo's government said the verdict was vindication of the guerrilla insurgency.

"This verdict is the strongest evidence that the Kosovo Liberation Army fought a just war for freedom and never committed the crimes of which we were unfairly accused," Kosovo Prime Minister Hashim Thaci, himself a former KLA commander, said in a statement.

Speculation is rife in Kosovo's media that Thaci plans to reshuffle his coalition government to bring in Haradinaj's Alliance for the Future of Kosovo, possibly ceding the post of prime minister to Haradinaj and seeking the presidency for himself.

Hardinaj's lawyer said the 44-year-old former nightclub bouncer would return to Kosovo on Thursday evening, "and with the consent of the people, will soon be resuming his rightful position as the political leader of the country".

"TIME FOR RECONCILIATION"

Serbia's nationalist President Tomislav Nikolic said the ruling was further evidence the U.N. tribunal for the former Yugoslavia was "formed to try the Serbian people" for the wars of Yugoslavia's collapse in the 1990s.

He warned it would "annul" progress made so far in fledgling EU-mediated talks between Belgrade and Pristina. Progress in the talks is a key condition of Serbia's further integration with the EU, which made Belgrade an official candidate for membership in March.

But Ivica Dacic, who holds the more powerful post of prime minister in Serbia's coalition government, has said the talks must go on, regardless of the Haradinaj verdict.

Kosovo was the last throw of the dice by late Serbian strongman Milosevic after he fomented wars in Bosnia and Croatia. NATO finally intervened in 1999 with 78 days of air strikes to halt a brutal Serbian crackdown in which almost a million ethnic Albanians were expelled.

"This is a big day for our nation," said Mustafe Bislimi, one of several hundred Kosovo Albanians who watched the verdict live on a big screen in the Kosovo capital, Pristina.

"This shows that his (Hardinaj's) war was right. We need him now more than ever."

Idriz Balaj, a former commander of a special KLA unit known as the Black Eagles, and Lahi Brahimaj, Haradinaj's uncle and a member of the KLA's general staff, were also acquitted in the retrial. Brahimaj has already served the six-year sentence he was given for torture at his first trial.

Judges ruled that ethnic Serbs and Albanians had been mistreated in camps controlled by the KLA, but found that nothing linked Haradinaj to these incidents.

On one occasion, Moloto said, Haradinaj had offered food and accommodation to an escaped detainee before returning him to his family.

(Additional reporting by Radosa Milutinovic in The Hague; Writing by Matt Robinson in Belgrade; Editing by Andrew Heavens)