The divided communities in Cyprus are to decide on the country's future after their leaders failed to agree on a UN plan to end the 30-year split.
Mr Annan called the plan a chance for peace on the divided island
UN Secretary General Kofi Annan presented his revised plan after talks in Switzerland with Greek, Turkish and Cypriot representatives.
The UN plan will be put to both sides in referendums on 24 April.
If either side says no, EU membership will in effect apply only to the Greek part when Cyprus joins on 1 May.
"Unfortunately it was not possible to reach an agreed settlement," said Greek Prime Minister Costas Karamanlis.
"It is now up to the people of Cyprus to reach a decision and I hope they will do this with clear thought and vision."
Mr Annan announced the plan in the Swiss mountain resort of Buergenstock, saying Cypriots now had a choice between the UN settlement or no settlement at all.
"Let us seize this chance for peace in the united republic of Cyprus."
Reporting from the talks, the BBC's David Bamford says Mr Annan made only limited progress.
But EU Enlargement Commissioner Guenter Verheugen was upbeat, saying the search for a settlement "certainly has not failed".
He also praised Turkey's "very constructive and co-operative role in the negotiations".
Greek Cypriots are especially opposed to two provisions in the plan:
reduced numbers of Greek Cypriot refugees who will be allowed to return to their own homes in the north,
- the continued presence of Turkish troops in Northern Cyprus.
Cyprus has been split into the Greek Cypriot-controlled south and the occupied north since Turkey invaded in 1974, following a short-lived coup by supporters of union with Greece. The breakaway north is only recognised by Turkey.
ORIGINAL UN PLAN
A united Cyprus run as two Swiss-style cantons
Right of return for Greek Cypriots
Symbolic, alternating presidency
While many on the Turkish side have embraced the UN proposals, even the veteran Turkish Cypriot leader Rauf Denktash - who boycotted the talks - said he would campaign for a no vote.
Turkey's Prime Minister Recep Tayyip Erdogan voiced support for the referendum, while remaining enigmatic on the plan itself, our correspondent reports.
"We set out on this road with goodwill. And we want to see the result of this."
Mr Annan has acknowledged that peace is only possible on Cyprus if the Turkish Cypriots can be assured there will not be a sudden rush of majority Greek Cypriots into their northern enclave, says our correspondent.
The plan envisages a federation of two states - one Greek and the other Turkish - with a loose central government, on the Swiss model.
It allows fewer refugees to return to the Turkish north and specifies that Turkey will be allowed to maintain a military presence on the island indefinitely.
The 9,000-page document was presented to the negotiators at a formal ceremony on the shores of Lake Lucerne earlier on Monday.
It contained several revisions to the original plan:
- A reduction in the number of Greek Cypriot refugees allowed to return to northern Cyprus from 21% to 18% of the Turkish Cypriot population
- A reduction in the size of Turkish Cypriot territory by 7% to 29% of Cyprus, allowing some 120,000 Greek Cypriots back to their homes under Greek
- Restrictions on property rights for Greek Cypriots in the north until the region approaches the living standards of the south
- Twenty-four seats for Turkish Cypriots in the 48-seat Senate, with the possibility of Greek Cypriots representing northern Cyprus removed
- No insistence on the withdrawal of all Turkish troops before Turkey joins the EU.