By Robin Emmott and Adrian Croft
BRUSSELS (Reuters) - European Council President Donald Tusk said on Friday he was ready to discuss the "mission impossible" of changing the European Union's treaty to accommodate the British prime minister's calls for radical reform of the 28-country bloc.
Tusk, who chairs EU summits and represents EU governments in managing the bloc, has made plain the difficulty of accommodating David Cameron's promise to renegotiate Britain's relationship with Europe, saying in a recent interview the demands were akin to the 'Mission: Impossible' action movies.
"I want to help find a solution that addresses the United Kingdom's concerns," Tusk, a center-right politician like Cameron, told a news conference following a summit in Brussels, appearing to back track from his earlier comments.
"If it's necessary we can also discuss about so-called 'mission impossible' with goodwill to win in the end," he said, referring to amending the EU treaty.
Cameron has promised that, if he wins a May election, he would renegotiate Britain's relationship with Europe on issues such as immigration and then hold a referendum by 2017.
He hopes that by doing so, he can convince Britons, who feel the European Union institutions have become overbearing and pernicious, not to quit the world's largest trading bloc.
Speaking at the same time in a separate news conference, Cameron took up the metaphor, saying that the star of the Mission: Impossible movies, Tom Cruise, "normally prevails in the end. He's a little bit smaller than me, but I hope to be just as effective."
Cameron said that if re-elected, he would have a mandate for change that would be difficult to ignore in Brussels.
But the issue is sensitive for Tusk because one of the Cameron's principal demands is to curtail immigration from eastern Europe when Poland is the biggest source of Europeans moving to Britain.
Cameron did not directly address the issue on Friday, saying only that he wanted "widescale change to the rules on welfare and benefit" in a reference to popular sentiment that EU immigrants come to Britain to scrounge off the state.
He also said he wanted Britain exempted from the EU's guiding aspiration of "ever closer union" although that is not a legal requirement in the bloc's Lisbon Treaty.
Changing the Lisbon treaty will be difficult, even with Tusk's support, however. Cameron will need to convince a majority of his European peers to support examining the idea. Then there would be a convention to draw up the changes, to be ratified by all EU governments.
(Editing by Dominic Evans)