By Matt Robinson

BELGRADE (Reuters) - Serbia's Orthodox Church warned on Saturday against a 'betrayal' of Kosovo, piling pressure on the ruling coalition as it weighs whether to cede the country's last foothold in its former province in exchange for talks on joining the European Union.

The appeal by the patriarch comes before a Tuesday deadline for the government to tell the EU whether it accepts a plan to tackle Kosovo's ethnic partition between its Albanian majority and a small northern pocket populated by Serbs.

Serbia considers Kosovo, steeped in history and myth, the cradle of the Serb nation and its Orthodox Christian faith.

But Albanians are its 90-percent majority, many of them Muslims. They broke away in war in 1998-99 and Kosovo finally declared independence in 2008, but Serbia still has a fragile hold on a northern pocket where some 50,000 Serbs live.

Rejection of the plan could cost Serbia a coveted place at talks on joining the EU, a process that would drive reform and help lure investors to the struggling Serbian economy, the biggest in the former Yugoslavia.

"The most important pre-election and post-election promise made by you, who received a mandate from the people to steer the Serbian ship in these tumultuous times, was that you would never, under any circumstances, surrender, betray or sell out Kosovo, historical Old Serbia," Patriarch Irinej said in a letter carried by the state news agency Tanjug and published on the church's website.

It was addressed to Serbia's nationalist president, Tomislav Nikolic, Socialist Prime Minister Ivica Dacic and the leader of the largest party in the coalition, Deputy Prime Minister Aleksandar Vucic.

ETHNIC PARTITION

"No Serbian leadership, including today's, has the right or mandate to agree to the kind of conditions that no other EU candidate country has been set. The entry ticket is too expensive," the letter said.

"Serbia must not agree to pay such a price upfront for goods that might never be delivered."

Serbia is under intense pressure to give up the northern pocket of Kosovo, an ethnic partition that the EU says must end if Belgrade is to make progress towards eventual membership of the bloc.

The Orthodox Church is highly respected in Serbia, gaining in influence as communism gave way to nationalism with the bloody breakup of federal Yugoslavia in the 1990s.

EU-mediated talks between Kosovo and Serbia broke up last week without result. EU foreign policy chief Catherine Ashton is due to report back to the EU on April 16, with Serbia's chances of starting accession talks this year riding on a deal being in place by then to tackle the partition.

Prime Minister Dacic is to meet parliamentary caucus leaders on Sunday, Tanjug reported, before chairing a cabinet meeting on Monday.

Serbia lost control over Kosovo in 1999, when NATO launched 11 weeks of air strikes to drive out Serbian forces accused of killing and expelling ethnic Albanians as they tried to crush a guerrilla insurgency.

Belgrade retained de facto control over the Serb-populated north centered on the divided town of Mitrovica.

Mired in recession and desperate for the economic boost of EU accession talks, Serbia has offered to recognize the authority of Pristina over the north, but wants autonomy for the Serbs living there.

Details of the latest proposal are not known, but Serbia has made clear it falls well short of the broad autonomy it sought. The decision has the potential to shatter the ruling coalition and could trigger violence in Kosovo.

(Additional reporting by Aleksandar Vasovic; Writing by Matt Robinson, edited by Richard Meares)