The Prime Minister of Kosovo, Ramush Haradinaj, has been charged with war crimes by the United Nations tribunal in The Hague. Mr Haradinaj has resigned and indicated that he will fly to the Netherlands tomorrow to face the charges.

Forewarned of the charges, Nato flew a detachment of 500 British soldiers into Kosovo last night to bolster the United Nations (UN) protection force, Kfor, which is keeping the peace in the country.

The former prime minister, a charismatic and comparatively young politician, is a member of Kosovo's ethnic Albanian majority, which struggled against the country's Serb overlords in the last decade.

Details of the indictment against Mr Haradinaj have not been made public, but the UN court has investigated him over his role as a district commander of the Kosovo Liberation Army, in the 1998-99 guerrilla war against Serb forces.

During the period dozens of Albanians suspected of collaborating with the Serb authorities and Serb families living in isolated farms were rounded up, tortured and killed.

"I resigned today from the post of prime minister and I've transmitted my resignation to the Kosovo president," said Mr Haradinaj, in a statement. "I am innocent," he added, although he refused to discuss the charges against him.

It is feared that today's news may reopen the ethnic fault-lines which last March led to a massacre in which at least 22 people died. Then, UN troops were forced to protect Serbs as organised gangs of armed ethnic Albanians attacked houses and churches throughout the province, in what appeared to be a co-ordinated and pre-planned act of provocation.

Today, the UN has raised its threat level and Nato sent extra troops to Kosovo in case of renewed ethnic unrest.

"Five hundred British troops flew into Pristina on Monday night," said Captain Sarah Erikson, a spokeswoman for Kfor. "There have been indications the security situation is not as good as we want it to be."

A Ministry of Defence spokeswoman confirmed that 500 soldiers of the First Battalion, Royal Green Jackets, had been sent to Kosovo last night. The British troops had been on standby as the alliance's current high readiness battalion in its operational reserve force.

It is understood that Nato requested the British troops up to four days ago, but the MoD spokeswoman declined to say whether the arrival of the Royal Green Jackets was linked to the UN war crimes charges.

"We are approaching a number of important local anniversaries, including the March 17 violence that took place last year," the spokeswoman said. "We are not in a position to comment on the international war crimes tribunal. The Royal Green Jackets will be under the command of Kfor."

Mr Haradinaj has been Prime Minister of Kosovo's coalition Government for only three months, taking up the post on December 4. He is the leader of the Alliance for the Future of Kosovo, the smaller of the two parties in the coalition. The elections were overwhelmingly boycotted by Kosovo's Serb minority.

Controversy surrounded Mr Haradinaj even before he took power. He was questioned in November by investigators sent by Carla del Ponte, the UN war crimes tribunal's chief prosecutor.

At the time, Javier Solana, the EU's foreign affairs spokesman, gave warning against his appointment: "If in the end the Prime Minister is somebody who has to go to The Hague, he may not be the most appropriate person."

Mr Haradinaj is also wanted by Serbia on 108 counts of alleged war crimes against Serb civilians during the war in Kosovo. The authorities in Belgrade have vowed to arrest him if he ever sets foot in Serbia, where he is accused of committing 67 murders, ordering 267 others and kidnapping some 400 during the war.

While technically still ruled by Serbia, Kosovo is in practice being directly governed by the UN. There are 20,000 UN peacekeepers in the country under the banner of Kfor, and a further 5,000 international police officers patrolling the fragile truce between the country's embattled communities.

Nick Hawton, The Times's Kosovo correspondent, said: "Mr Haradinaj is likely to go quietly, but the question is, how will his hardline supporters react to their great war hero and leader being arrested by the war crimes tribunal?

"All eyes will be on his heartland in western Kosovo, and with the first anniversary of the unrest coming up, this is a very sensitive time."

In western Kosovo, Germany was also reinforcing its peacekeeping contingent, while the protectorate's UN administration warned non-essential personnel to stay off the streets.

The news is an enormous setback for the political future of Kosovo, which is facing a UN review this summer of how far it has gone along the road of meeting the required standards of democracy, rule of law and protection of minority groups, such as the Serbs, before talks can begin on the country's long-term status.

Kosovo's Albanian majority has been fervently hoping that the UN will give a positive assessment, so that their country - dominated by the Serb empire until 1389, then by the empire of the Ottoman Turks until 1918, and since then claimed by Serbia again - can finally move towards independence.

Mr Haradinaj's resignation is understood to mean that the government must stand down, throwing the country's future into fresh uncertainty and losing precious time in the run-up to the summer review.

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