That's the date when Occupy Central will end an unofficial citywide referendum in which three shortlisted proposals for the 2017 chief executive election will be put forward to the public to vote on.

The shortlisting of the three proposals - by Scholarism, People Power, and the Alliance for True Democracy - has split the pan-democrats, with the Democratic Party condemning the two radical groups People Power and the League of Social Democrats for reneging on promises to support the alliance's proposal.

The shortlist also alienated moderates who say their proposals were screened out.

The Democratic Party has vowed to continue to promote the alliance's proposal, under which the nominating committee would approve nominations from the public and political parties as a formality.

Civic Party leader Alan Leong Kah-kit said he would do his best to persuade the Democratic Party to stay in the alliance. He said the alliance, which formed in March last year, was needed as a platform to display pan-democrats' solidarity.

The idea of dissolving the alliance was first floated by its convenor Joseph Cheng Yu-shek on Wednesday.

"The alliance is facing a crisis," Cheng said.

"But we will try our best to do the job until the referendum."

The development came as Chief Secretary Carrie Lam Cheng Yuet-ngor yesterday urged citizens to think carefully before deciding whether to support any of the three proposals on offer in Occupy Central's vote.

Lam (pictured) expressed doubt over whether any of the proposals could be a foundation for reform, as each called for the public to be given the right to nominate candidates - a demand Beijing has consistently rejected.

She said it was "difficult" to see the proposals as "compatible with the Basic Law and the decision by the National People's Congress' Standing Committee".

"The public needs to consider whether [the vote] offers real choices and whether they should participate in it," she said.

Justice Secretary Rimsky Yuen Kwok-keung added that the polling exercise could not turn something "illegal" into being "legal".

Beijing has said that the Basic Law requires the official nominating committee to have the sole right to propose candidates.

Occupy Central's advocate Benny Tai Yiu-ting said Lam's remarks were an acknowledgement of the pressure likely to be exerted by the June 20-22 vote on its future reform proposal.

The Education Bureau said its minister Eddie Ng Hak-kim - who on Wednesday called on schools not to encourage pupils to take part in Occupy Central - was only "reminding" the schools, not "exerting pressure" on them.