Guardian Media Group is looking at the future of the 200-year-old Sunday paper, with options being considered thought to include its closure or transformation into a weekly magazine.
In the most recent set of ABC figures, which cover the period before a concerted 'Save The Observer' campaign was started, the paper's circulation was down 7% year on year in July to 398,330 copies.
Union members have said that they will try to save The Observer and will also resist compulsory redundancies.
Following a vote, a motion was passed declaring: "The Guardian and Observer NUJ chapel believes that the survival of the Observer as the world's oldest Sunday newspaper is essential for the protection of pluralism and diversity in the British media and calls on the Scott Trust and GNM/GMG management to reject proposals to drop the title, downgrade it to a weekly magazine or undermine its character as an independent and competitive Sunday newspaper."
A nominal offer for the title , but Guardian Media Group has said the bid is "unwelcome" and "opportunistic".