Progress on the issue has been stalled since Greek Cypriots rejected a UN-proposed reunification plan in 2004.
Taye-Brook Zerihoun, head of the UN mission on the island, made the announcement after talks between President Demetris Christofias and Turkish Cypriot leader Mehmet Ali Talat.
"Having made their final review, the leaders decided to start their fully fledged negotiations on September 3, 2008, under the good offices mission of the UN secretary general," he said.
"The aim of fully fledged negotiations is to find a mutually acceptable solution to the Cyprus problem, which will safeguard the fundamental and legitimate rights and interests of Greek Cypriots and Turkish Cypriots."
He added that any agreed solution would be put to separate, simultaneous referendums in both communities.
Cyprus has been divided since 1974 when Turkey invaded the north in response to a military coup on the island that was backed by Athens.
