The underground reservoir is covered in vegetation
Campaigners will march to Reading civic offices later calling on a reservoir to be protected from development.
Plans to build 100 homes at the Bath Road site were rejected by councillors last year over claims it would have had a detrimental impact on wildlife.
Landowner Thames Water had wanted to sell the site to developers with planning permission for houses.
The Save the Bath Road Reservoir group will deliver more than 200 letters calling for the site to be protected.
Members of the public have until Tuesday to submit applications to be considered in the council's draft plan on which areas should be protected from development.
Save the Bath Road Reservoir said: "This document identifies sites both for development and protection.
"We feel that the reservoir site is worthy of protection, and should be designated as a local nature reserve, a wildlife link, an educational resource for local schoolchildren or similar."
The group said the 5.4 acre (2.2 hectare) underground reservoir, which is covered in vegetation, was a "green lung" for the town.
Councillors listed 12 reasons when they threw out the plans last October.
Thames Water said it was "disappointed" with the council's decision but was looking at ways to progress the plans.