ENGIE The letter, addressed to European Commission President Jean-Claude Juncker, reignites a debate that has pitched dominant EU member Germany against central and eastern European nations.
If the project goes ahead, Nord Stream-2 would generate "potentially destabilising geopolitical consequences", the letter says.
The European Commission said it could not confirm receipt of the letter.
"The Nord Stream-2 project that is currently under preparation can pose certain risks for energy security in the region of central and eastern Europe," the letter, dated March 7, says.
"It would strongly influence gas market development and gas transit patterns in the region, most notably the transit route via Ukraine."
Russia's gas export pipelines have become intensely political because of the conflict in Ukraine.
Ukraine is the transit route for about half of the gas Russia's Gazprom sells to the European Union, which relies on Russia for roughly a third of its supplies.
The copy of the letter seen by Reuters is signed by the prime ministers of the Czech Republic, Estonia, Hungary, Latvia, Poland, Slovakia and Romania and the president of Lithuania.
Two EU sources, speaking on condition of anonymity, said Croatia had also signed a later version of the letter, but this could not immediately be confirmed with the Croatian government.
EU leaders meet for summit talks in Brussels on Thursday, when energy security is expected to be discussed.
Nord Stream-2 generated heated debate at summit talks in December following another letter sent to the Commission.
European Council President Donald Tusk of Poland said then it would be up to the Commission to make a final decision on legality. However, in his view Nord Stream-2, backed by Gazprom, E.ON, Wintershall, Shell, OMV and Engie, flouted EU rules.
Gazprom and some utilities say Nord Stream-2 is the most pragmatic solution to shoring up Europe's energy security. The Commission has said it is assessing its legality.
German Vice Chancellor Sigmar Gabriel travelled to Poland early this year where he promised the new pipeline would go ahead only if Russia did not cut off Ukraine and eastern European gas flows.
Gazprom already sends gas to Germany across the Baltic Sea via the Nord Stream pipelines. Nord Stream-2 would add a second twin subsea pipeline from Russia to Germany, doubling capacity to 110 billion cubic metres per year.
(Additional reporting by Barbara Lewis and Foo Yun Chee in Brussels; Writing by Barbara Lewis; Editing by Dale Hudson)
By Andrius Sytas
