The 28 EU leaders were initially due to begin a regular two-day meeting on June 23, the same day as the so-called Brexit vote in Britain.
"The June meeting of the European Council confirmed for 28-29 June," Preben Aamann, a spokesman for EU president Donald Tusk, said in a tweet.
Aamann then told AFP that the decision was taken "to avoid an overlap with the UK in/out referendum."
Prime Minister David Cameron promised the in-out EU referendum before Britain's general election last year.
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The 28 EU leaders meet regularly for two-day summits throughout the year to review outstanding issues and set broad policy outlines for the bloc.
The Brexit issue and Cameron's reform demands dominated their February 18-19 summit.
Opinion polls have been mixed overall but a survey in The Observer newspaper at the weekend put support for Britain to leave the EU at 43 per cent, against 39 per cent wanting to stay.