Two officers from the Metropolitan Police Special Operations Department in London set off for the Kosovan capital, Pristina, on Saturday to set up headquarters for a British forensic team.

It will pass its evidence on to the International War Crimes Tribunal in the Hague.

The two officers are paving the way for a team of about 15 UK experts due to arrive in Kosovo next week, and will decide what equipment will be needed. Mr Straw said the team of forensic experts "will document and prepare evidence of ethnic cleansing in Kosovo".

"Their forensic skills will be used to prepare evidence for the War Crimes Tribunal to bring the perpetrators of these vile crimes to justice," he added.

The Foreign Office's war crime co-ordinator David Gowan has access to some of the latest intelligence information.

He said: "The scale of the criminality is enormous. The number of people who have been murdered is greater than we think by far.

"It is going to be chilling."

Mrs Arbour: Said to be feeling the strain

Defence Secretary George Robertson told BBC One's Breakfast With Frost that UK troops must hand any evidence of war crimes over to the tribunal.

He said some "truly horrible" things have taken place in Kosovo, and those responsible must be brought to justice.

Britain is one of several Nato countries sending forensic experts to Kosovo. The US Federal Bureau of Investigation (FBI) has also sent its first group of 25 people to Kosovo.

FBI Director Louis Freeh said it has sent the same teams who went to Kenya and Tanzania after the embassy bombings.

Based on their findings, a second, larger deployment will go in the next two weeks for a detailed investigation.

The experts will have to try and gather as much evidence as possible before winter sets in, burying the evidence under layers of mud and snow.

Arbour leaves post

A key figure under investigation will be Yugoslav President Slobodan Milosevic. He was indicted for crimes against humanity in May by the chief prosecutor of the UN war crimes, Louise Arbour.

Mr Robertson said: "There is no way that Milosevic can run on a permanent basis.

"He is indicted, and I believe that one day he will face justice, and so he should."

Justice Arbour announced she was leaving to join Canada's Supreme Court only two weeks after indicting President Milosevic.

Canadian Justice Minister Anne McLellan said Mrs Arbour was delaying taking up her new post until 15 September to ensure "continuity in the work of the war crimes tribunal".

A new chief prosecutor will be chosen by the UN Security Council, which created the tribunal in 1993 to investigate war crimes in the former Yugoslavia.