The underground reservoir is covered in vegetation
A decision to reject plans by Thames Water to build almost 100 new homes at a Berkshire reservoir is to be explained by councillors at a meeting.
Thames Water had wanted to sell its Bath Road site in Reading to developers with planning permission for the homes.
Opponents argued the 5.4 acre (2.2 hectare) underground reservoir, which is covered in vegetation, was a "green lung" for the town.
Councillors voted unanimously in October to reject the plans.
Thames Water said at the time that it was "disappointed".
It says it has a duty to its customers to get the maximum value from selling the land, once it becomes no longer needed early next year.
The council received more than 400 letters of protest in opposition.
Its planning committee will meet at 1830 GMT to publicly give its reasons for the rejection.
Campaigners say the vegetation on top of the reservoir is a "green lung"
Campaigner Mel Woodward, of the Save the Bath Road Reservoir group said: "It is in the best interests of everyone... that these reasons cover all of the planning grounds that justify why the application is unsuitable for the site.
"Because then if Thames Water appeal, the government will then not be able to overrule it, and we won't end up with a potentially similar or worse development."
The group said the vegetation covering the underground reservoir had been untouched for decades, allowing wildlife such as deer, foxes and badgers to flourish.
They want to see Thames Water turn the area into an educational resource for local schools.
A previous planning bid for the site by Thames Water was turned down last year.
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