On his first foreign trip since winning general elections earlier this month, Cameron is set to meet some of Europe's leading figures as he seeks to secure reforms over issues that include immigration.
The Eastern Partnership summit in the Latvian capital, which kicks off on Thursday, focuses on how the 28-member bloc should reconcile its commitment to six former Soviet states with its relationship with Russia. But it will also signal the start of Cameron's renegotiation push, a painstaking process likely to last for months which the British leader says will require EU treaty change.
"It's the first overseas visit for the prime minister since the election and it's his first opportunity to have some discussions with partners about the way he wants to reform the European Union, renegotiate the UK's relationship with it and the referendum," Cameron's spokesman told reporters.
"The package of measures outlined will need treaty change," he said.
Under pressure from eurosceptics, the British prime minister promised in 2013 to hold a referendum on whether Britain should leave Europe by 2017 if he won the general election.
Having secured victory on May 7 with a surprise though slender majority, he is now under pressure to hold the vote sooner rather than later.
Some senior members of his centre-right Conservative party have called for it to take place in 2016, along with Andy Burnham, a prominent contender to be next leader of the main opposition Labour party.
Nigel Farage, leader of the anti-EU UK Independence Party which has been riven with splits since winning only one seat at the election, believes the referendum will be held in May next year. Farage warned of "a great stitch-up where the European Commission and the European Council are seen to give some cosmetic concessions to Britain".
Cameron's spokesman said that the plan was to hold it "by the end of '17 - if we can do it earlier then we would look at that".
Cameron says he will campaign to stay in Europe in the referendum as long as he can secure the reforms he says are necessary.
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