An environment group is celebrating after winning a High Court battle to protect a home to rare mushrooms, which is being proposed for new housing.
A number of different types of wax caps have been found
Luxury homes have been planned at two Cardiff reservoirs but grassland fungi there led to them being named Sites of Special Scientific Interest (SSSIs).
Landowners Western Power Distribution lost its challenge to the Countryside Council for Wales (CCW) decision.
But a High Court judge ruled that it was an "important site".
The decision means the plans to develop Lisvane and Llanishen reservoirs for housing have effectively been blocked, although the final decision rests with the Welsh assembly government, who are overseeing the planning process.
The CCW designated the site on the embankments of the reservoirs to protect what they claim is an internationally important population of fungi.
But Western Power Distribution claimed the CCW had acted in a way which was "premature, arbitrary and unfair."
However on hearing the evidence, Mr Justice Collins agreed with the CCW.
"The decision was in the end based on the acceptance that the number of species found in Llanishen justified the notification," he said.
"Whether or not it was the best site was not the issue. I have no doubt that the CWW was entitled to conclude as it did.
"There was ample evidence before it that the number of species made this an important site."
He said the site was of international importance because of the large number of fungi species on it.
Earlier in his judgment he commented that the reservoirs which were created between 1860 and 1880 were no longer needed and that the owners, Western Power, now wanted to build houses there.
However, the court had been told that the CCW decision to designate the embankments as an SSSI was based on their view that the site was the sixth best for grassland fungi in Wales and as such a site of "international importance."
They had identified 28 species of Hygrocybe (or 'waxcap') fungi on the site. This put it over the benchmark for international importance which is set at more than 22 species.
However, Western Power said the CCW had acted too quickly in protecting this site and failed to follow the correct process.
But, the CCW argued that the site was special by reason of the wide range of grassland fungi it is known to support.
It said that if further information came to light indicating that the site was not of special interest, then it could be de-notified but until then it should be protected.
And it added that the site had been designated because it was considered to be of special interest, not because of any threat from development.
The ruling has "delighted" campaigners who have been fighting the plans to develop the area into housing, which was first put forward in 2002.
Landowner Western Power Distribution wants to reduce the 60-acre Llanishen reservoir in size, and build luxury homes on its banks.
But local people have been campaigning against the plans and city planners also rejected the proposals.
Ted Thurgood from Llanishen Reservoir Action Group said: "There will be great celebrations in north Cardiff tonight but this is not the end of the battle and we will be fighting on."
Western Power declined to comment.