The largest Greek Cypriot political party has come out against the UN's plan to reunite the island.
Greek Cypriots could scupper the unification deal
The communist Akel party withdrew its support after failing in attempts to have the poll delayed.
Opinion polls suggest 65% of Greek Cypriots already reject the plan, as it goes to referendums in both communities this weekend.
The setback comes after Russia vetoed a United Nations resolution designed to encourage a "yes" vote.
The Akel party said its decision came because it failed to get guarantees from the UN that Turkish troops would be removed from the north of the island following reunification.
Last campaigning day
"I'm sorry to say the response (from the UN) was not what we expected," said Dimitris Christofias, secretary-general of the party.
Thursday is the last day of campaigning for the separate Greek and Turkish Cypriot votes on the plan, drawn up by UN Secretary General Kofi Annan.
If both sides support the proposals, a united Cyprus will join the European Union on 1 May.
If either side rejects the plan, membership will in effect apply only for the Greek Cypriots.
Sixty percent of Turkish voters north of the UN-manned separation line are ready to adopt the deal, according to opinion polls, while Greek Cypriots look likely to scupper it.
Diplomats and politicians reacted with despair as they saw support draining for a template they hoped would end 30 years of division.
The EU's Gunter Verheugen said the Greek Cypriot government had cheated the EU by pretending to support the unification plan while in reality campaigning against it.
Mr Verheugen said the government "had taken him for a ride", and he directly criticised the Greek Cypriot leader, Tassos Papadopoulos.
At the UN, Russia used its first veto since 1993 to block a resolution on proposed security arrangements should the islanders vote for reunification.
Russia's deputy ambassador to the UN, Gennady Gatilov, said his country saw the resolution as an attempt to influence Saturday's twin referendums.
Russia has been a staunch supporter of Greek Cypriot
authorities, who claim the UN plan is too weak.
But the Turkish Cypriot leader Rauf Denktash - who is a strong opponent of the UN plan despite public opinion - was delighted.
"God bless Russia. They have prevented a catastrophe," he said.