The Latest on EU summit on changes demanded by Britain (all times local):

3:10 p.m.

European Union President Donald Tusk says Thursday's summit of the 28 EU leaders to decide on the reforms that British Prime Minister David Cameron is calling for will be one of the most important the bloc has seen.

With many core issues still outstanding going into the two-day summit, Tusk said hours ahead of the opening summit session: "One thing is clear to me though. This is a make or break summit, I have no doubts."

Tusk said that both sides were still "in the middle of still very difficult and sensitive negotiations," as the summit center not only started filling with leaders but also legal experts who would have to pour any decision into binding legal texts.

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2:30 p.m.

Prime Minister David Cameron says he'll be "battling for Britain" but still faces hurdles in his talks on a deal with the European Union.

Arriving at an EU summit in Brussels, Cameron said he was fighting to close a deal by Friday, but "it's going to be hard."

He said that "if we can get a good deal I'll take that deal, but I will not take a deal doesn't meet what we need. I think it's much more important to get this right than to do anything in a rush."

Cameron hopes the 28 EU nations can strike agreement on the changes Britain seeks to its relationship with the bloc, so he can call a referendum on U.K. membership for as early as June.