The letter, dated Oct. 16, was obtained yesterday by The Associated Press.
One prominent member of the European Parliament said the Chinese threat was counterproductive.
"Heavy-handed Chinese state lobbying to influence the outcome of the Sakharov prize winner only plays into the hands of critics and reinforces the case for Hu Jia," said Graham Watson, the leader of the EU Liberal group.
Song's warning comes ahead of an EU-Asia summit in Beijing tomorrow and Saturday, during which French President Nicolas Sarkozy, the EU president, wants to persuade China and other Asia nations to join in an overhaul of the world financial system.
Hu is a brash dissident who chronicled the arrests and harassment of other activists before he was sentenced in April to 3 1/2 years in jail in China.
He started out fighting for the rights of HIV/AIDS patients, but his scope expanded after the government gave little ground and he began to see China's problems as rooted in authorities' lack of respect for human rights.
Joining Hu on the shortlist are Belarus opposition leader Alexander Kozulin and Abbe Apollinaire Malu-Malu, who guided the Democratic Republic of Congo through its first elections in 50 years in 2006 as chief of the nation's electoral commission.
Hu had also been considered a front-runner for this year's Nobel Peace Prize, which drew Chinese criticism, too. That prize went to former Finland President Martti Ahtisaari.
