Merkel penned the letter along with Christian Social Union leader Edmund Stoiber, timing it to hit the desk of center-right leaders just before an informal meeting on Thursday of EU foreign ministers, where EU accession negotiations with Turkey will be discussed.
Turkey is expected to open membership negotiations with the EU on Oct. 3.
"We are fully convinced that accepting Turkey would overburden the EU politically, economically and socially and would endanger the European integration process," the letter, addressed to 11 European leaders, said.
The letter cited "the continuing refusal of Turkey to recognize the Republic of Cyprus, a member state, under international law" as an obstacle to Turkish membership, and also noted "still-significant problems in upholding and imposing human rights."
Merkel and Stoiber underlined that the EU was interested in a close connection between Turkey and the European Union, but "we ask you to make clear that the framework of the negotiations also includes the perspective of a privileged partnership with Turkey."
A copy of the letter also went to the current EU president, British Prime Minister Tony Blair, and EU Commission President Jose Barroso.
The response from Joschka Fischer, German foreign minister, was heated.
"The attitude of Ms. Merkel and Mr. Stoiber is dangerously blind considering the promises that have been made by German governments over the past 43 years, from Adenauer to Kohl," Fischer told the dpa news agency.
Won't "overplay" Turkey
On Monday this week, German conservatives said they would not let the Turkey issue hijack the campaign in front of the country's general election, scheduled for Sept. 18. But the interior minister of the state of Bavaria, Günther Beckstein, said Turkey had become an election issue and "should not be neglected."
"Turkey would now be the second most important country in a European Union and in a few years … Turkey would be the leading nation, more than France, Italy or Austria, the Netherlands, Portugal and Greece together," he said.
Beckstein said the rejections by France and the Netherlands of a new EU constitution earlier this year showed that EU citizens were not ready for another significant expansion of the union.
Wolfgang Schäuble, foreign affairs adviser in Merkel's pre-election team, echoed those sentiments in an interview with the International Herald Tribune. He also raised questions about Turkey's identity.
"A part of Turkey is Europe, just like Russia," he said in the interview. "But a far bigger part of both Turkey and Russia is definitely not in Europe. That is why Russia could never really integrate into the EU."
The position puts a possible Merkel government at odds with the current EU position as well as with that of the United States, with which Merkel has said she wants a closer relationship.
The US considers Turkey of great strategic importance and would like to see the country taken into the European fold. During a trip last month to Washington, Schäuble said he explained his party's position to US Secretary of State Condoleezza Rice.
"I think Condoleezza Rice was satisfied with my arguments," he said.
It is not the first time Merkel has pushed for an EU relationship with Turkey which falls short of full membership. She brought the topic up a year ago, just three weeks before the EU Commission released a report on Turkey and a few days before regional elections in two eastern German states.
Then there was disagreement over the proposal even within her own party. Fellow Christian Democrat and former defense minister Volker Rühe reminded Merkel that the conservative government of Helmut Kohl had in 1997 signed an agreement laying out Turkey's official EU candidate status. He said closing the door would destabilize the country and could strengthen anti-European sentiment there.
In Brussels, Merkel's approach met with astonishment. Jean-Claude Juncker, Prime Minister of Luxembourg and then-EU president, distanced himself, saying Turkey had enjoyed "candidate status" for years and it was too late to offer it a privileged partnership.
CDU foreign affairs expert Schäuble said despite the position of his party, a Merkel government would not stop the beginning of accession negotiations between the EU and Turkey. He said Germany would stand by the European Council's decision that the results of negotiations should be kept open."The negotiations could last for at least 10 years," he said. "Things could be very different in 10 years' time."