Cameron 'must make case' for EU membership, says Tusk
European Council President Donald Tusk has said he wants newly re-elected British Prime Minister David Cameron to make the case for EU membership.
Mr Tusk said he was "deeply convinced that there is no better life outside the European Union, for any country".
Mr Cameron has pledged to negotiate a "better deal" for the UK and hold a referendum on membership.
Leaders from across the globe have been congratulating him after he defied predictions to win a majority.
"I count on the new British Government making the case for the United Kingdom's continued membership of the European Union. In that I stand ready to help," Mr Tusk said in a statement.
"A better EU is in the interest not only of Britain but of every member state."
Analysis: Katya Adler, Europe Editor, BBC News
The words "political earthquake" have been translated into numerous European languages today, making front page news across the continent.
In Europe the vote means one thing - a referendum on Britain's membership to the EU. And that has the potential to create an earthquake of its own.
Mr Cameron has promised a referendum in 2017, and Brussels is hardly known for speedy decision-making. Expect quite some flexibility - few countries want to see the UK's burgeoning economy leave the fold.
But there will be limits, possibly over people's freedom to live and work anywhere in the EU.
European Commission President Jean-Claude Juncker said he was "looking forward to meeting Mr Cameron soon".
A statement from his office said he would examine any British proposals in "a very polite, friendly and objective way" but warned that key principles including the freedom of movement were "non-negotiable".
Offering his congratulations in a phone call, French President Francois Hollande invited Mr Cameron for talks in Paris on the EU and international issues.
Spanish Prime Minister Mariano Rajoy of the Conservative People's Party, tweeting in Spanish, called the victory a "deserved recognition of his decisive reforms".
Manfred Weber, who chairs the European People's Party grouping that Mr Cameron withdrew his party from, tweeted that the "ball is in Mr Cameron's court. He has to put his demands on the table. But EU freedoms will not be negotiable".
How world leaders reacted
- President Obama congratulated Mr Cameron on an "impressive electoral victory", praising the "special and essential relationship" between the US and UK
- Japan's Prime Minister Shinzo Abe called the win "splendid"
- The Indian Prime Minister Narendra Modi tweeted congratulations, and referenced a Hindi campaign slogan adopted by Mr Cameron to woo voters from an Indian background
- Israeli Prime Minster Benjamin Netanyahu tweeted: "I look forward to working with you on shared goals of peace & prosperity"
- Russia said it was watching results closely, but said it regretted that "relations are somewhat frozen at present"
"We Europeans must also start thinking about whether it is time for a larger Treaty reform," he added.
Aside from questions over Britain's future in the EU, Danish Prime Minister Helle Thorning-Schmidt said her husband Stephen Kinnock, the son of a former leader of the opposition Labour party, had "done so well" to win a seat.
Asked by Danish newspaper Berlingske who would be doing the commuting, she said: "I've always had a husband who works outside Denmark, and now he will do so in politics. This just means we will have even more to talk about, since we are both interested in politics."