(Adds full Commission quote)
By Gabriela Baczynska
BRUSSELS, June 8 (Reuters) - The head of the European Council, Donald Tusk, has waded into a dispute over a proposed new pipeline for Russian gas that pits Germany against eastern members of the EU, saying the plan would be harmful to the bloc's interests.
Poland, the Baltic states and others argue that Nord Stream 2 would increase the European Union's dependence on Russia's Gazprom, which already supplies about a third of the bloc's gas. Supporters say it will mean cheaper gas supplies for Europe.
"You know that my view about this project is negative," Donald Tusk, a former Polish prime minister who now chairs summits of EU leaders, wrote in a letter seen by Reuters on Thursday.
"It will not serve the best European interest," Tusk said in the letter, addressed to Jean-Claude Juncker, head of the executive European Commission which is preparing to negotiate with Russia on the project.
Opponents of the pipeline also note that it would bypass Ukraine, thereby harming the interests of the pro-Western government in Kiev at a time when Russia remains under EU sanctions over its annexation of Ukraine's Crimean Peninsula and its support for separatist rebels in eastern Ukraine.
Gazprom's European partners n the Nord Stream 2 project include Germany's Uniper, Austria's OMV and France's Engie.
The European Commission, comprised of 28 officials, one from each EU state, is working on a proposal for a mandate to launch negotiations with Russia on the pipeline to ensure it meets EU laws.
"I encourage you to be determined in demanding that all our rules stemming from the legal obligations ... as well as our political objectives are applied to this project in full," Tusk said in his letter to Juncker.
In commenting on the letter, which has not been made public, a spokesman for the Commission said on Tuesday: "The Commission is an institution based on law. We therefore cannot take arbitrary decisions favouring a project, or blocking another on the basis of preferences."
"The European Commission is now working on the legal framework that will ensure that the construction of such an important infrastructure does not happen in a legal void," said the spokesman, Margaritis Schinas. He said he hoped member states would endorse the proposal.
Three sources in Brussels said the Commission was likely to approve on Friday the negotiating mandate proposal, which would then have to be endorsed by EU states before any talks with Moscow start.
Two other officials said those countries opposed to the Nord Stream extension were working through their nationally appointed commissioners to make the mandate more demanding. This could delay its approval in Brussels even before the proposal reaches member states for further talks. (Editing by Gareth Jones)