German chancellor Angela Merkel threw a European lifeline to David Cameron after he unveiled the referendum which could see Britain walk out of the European Union.
Merkel indicated that Germany was willing to be flexible during negotiations about Britain taking back powers from Brussels.
Cameron announced a plan for an in/out vote in 2017 after Britain had negotiated a new settlement with the European Union.
The PM insisted he wanted Britain to stay in the bloc because he was confident summit talks would be a success and Britain would get what it wanted.
Merkel's comments suggested that talks could be fruitful. Germany is one of the most influential countries in the eurozone.
Merkel said: "Germany, and I personally, want Britain to be an important part and an active member of the European Union.
"We are prepared to talk about British wishes but we must always bear in mind that other countries have different wishes and we must find a fair compromise. We will talk intensively with Britain about its individual ideas."
Her comments contrasted with those of other senior French politicians. Foreign minister Laurent Fabius called Cameron's speech "dangerous" and said he opposed Britain "cherry-picking" which powers to take back and which to delegate to Brussels. "No to Europe á la carte," he said.
Integration more important than ever
Spain's foreign minister José García-Margallo insisted that European should focus on forging deeper ties, not loosening them.
He said: "[Britain] can't demand a renegotiation of the [EU] treaties to tear up what we have done or slow down the EU's cruising speed at a time when integration is more important than ever."
More supportive noises for Cameron's stance on European membership came from Holland, the country in which the prime minister originally intended to deliver his keynote speech, and Finland.
Finland's foreign minister Alex Stubb tweeted: "Cameron speech more constructive than expected. Like most of the economic principles. Disagree on [eurozone] deepening."