Serbia took a symbolic step towards joining the EU by signing an agreement on closer cooperation today as Britain announced it would send 600 troops to help to keep order in the breakaway province of Kosovo.

EU foreign ministers toasted the new pact with champagne but it was widely seen as a gesture timed to give a boost to pro-EU parties in the Serb general election on May 11.

The agreement is unlikely to come into force for some time because it requires full Serbian cooperation with the International Criminal Tribunal for Yugoslavia, which is demanding the arrest of key fugitives including former Bosnian Serb general Ratko Mladic and wartime leader Radovan Karadzic. Both have been on the run since the 1990s.

“We believe we are irreversibly on the road to EU membership,” said Vuk Jeremic, the Serbian foreign minister and a member of the pro-EU faction in the Government.

“This is absolutely not an empty shell, this is a great political step. This is going to be closely related to the voting that is taking place in Serbia on May 11. It is going to be a referendum on our membership in the European Union.

Nationalist politicians in Belgrade vowed to repeal the agreement if they seized power from pro-EU reformers in a poll triggered by Kosovo’s declaration of independence from Serbia in February.

The nationalists blame the EU for encouraging Kosovan ambitions and want to turn Serbia back towards Russia, a move likely to stoke tensions in the Balkans as its neighbours seek closer ties with the West.

Vojislav Kostunica, the nationalist Serb Prime Minister, equated the new pact with EU approval for Kosovo to break away, even though only 18 of the 27 EU nations have formally recognised independence.

“We will never allow anybody to sign for the independence of Kosovo in the name of Serbia and that is why today’s signature is not worth anything,” he said.

The agreement today could only be signed after a climbdown from Belgium and the Netherlands, which had wanted the final war crimes suspects under lock and key before taking any further moves towards membership.

But they were persuaded after much diplomatic arm-twisting that the signing ceremony would send a powerful message of goodwill and help influence Serb voters who are split between the pro and anti EU parties.

Jim Murphy, the Europe Minister, said: “It is for the Serbian people now to decide their future. Every nember of the EU has invited them to become part of this organisation. The Serbian people have got a choice to make whether they want to be part of that European future.”

British troops from Northern Ireland-based 2nd Battalion The Rifles will be sent for one month to reinforce Nato forces in Kosovo before the end of May, Des Browne, the Defence Secretary, announced today.

Tensions have been running high in the Serb-dominated north of Kosovo, where a riot roke out in March in which one UN peacekeeper died. The British deployment, due to end on June 30, was timed to coincide with the tense aftermath of the Serb elections as the UN prepares to hand over its policing responsibility to the EU.

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