UN Secretary General Kofi Annan has unveiled a revised UN plan for the reunification of Cyprus.
The deadline for EU membership is rapidly approaching
He presented it to Greek Cypriot and Turkish Cypriot leaders at a meeting currently taking place in Switzerland.
The two sides have until Wednesday to agree any changes before the plan is put to the two Cypriot communities in referendums in three weeks' time.
Pressure is on to reach a deal to unite the divided island before it joins the EU on 1 May.
Time running out
The 9,000-page document was presented to negotiators from Greece, Turkey and the two Cypriot entities at a formal ceremony on the shores of Lake Lucerne.
UN officials said Mr Annan's revised plan was still being written less than an hour before being handed over - only one copy per delegation was printed in time.
Addressing the four delegations seated around him, Mr Annan said that in presenting his plan there was a sense of destiny.
Details of the plan have not been made widely available, but a UN official told Reuters news agency the document contained major changes to the plan first presented more than a year ago.
Revisions included a proposed 5% reduction in land held by Turkish Cypriots and a reduction in the number of Greek Cypriots allowed to settle in Turkish northern Cyprus, the official said.
David Bamford, the BBC's correspondent at the talks, says it is the best chance by far for the Cypriots to put 30 years of enmity and division behind them.
But if no deal is reached by the end of the month, EU membership will in effect apply only in the Greek part of Cyprus.
UN peace plan for Cyprus
A united Cyprus run as two Swiss-style cantons
Right of return for Greek Cypriots
Symbolic, alternating presidency
In 1983, the Turkish-held area declared itself the Turkish Republic of Northern Cyprus, and is recognised internationally only by Turkey.
Along with the Turkish prime minister, Recep Tayyip Erdogan, and Greek Prime Minister Costas Karamanlis, delegations from the EU, Russia and the US are also attending.
Our correspondent says Mr Annan's main proposals are already known: the establishment of a loosely-federated state; and the return to northern Cyprus for some, but not all, of the 180,000 Greek Cypriots who left when Turkish troops occupied that part of the island in 1974.
Give and take
His blueprint envisages a power-sharing arrangement giving the Greek and Turkish communities large degrees of autonomy over domestic affairs.
But although the broad brush strokes of the plan are known, the Greek and Turkish communities will have just three days to consider the various annexes and supplements, which take up several thousand pages of text.
The two sides would "embark on two very heavy days and nights of give and take", said Greek Cypriot parliament speaker Dimitris Christofias.
"We are at a point where we cannot be optimistic or pessimistic - we have to be realistic," said Turkish Cypriot foreign minister Serdar Denktash.
Mr Annan has asked the delegations to take away this document and come back on Tuesday with further comments.
After Wednesday there can be no further changes and all sides have agreed to put the plan to the people of Cyprus in a simple referendum vote in April.
Our correspondent says both sides have placed a remarkable trust in Mr Annan, giving him the power to fill in any gaps in the proposals on which they cannot agree.