3m of water from the top of Llanishen Reservoir is being drained into a stream
An energy company has been urged to stop draining a disused reservoir in Cardiff which campaigners are fighting to protect.
Western Power Distribution has been trying for nine years to build 300 homes at its Llanishen Reservoir site.
But it has failed to gain permission after numerous court cases and appeals.
It says it has now started to drain top water from the reservoir for inspection work but the Environment Agency insists it is not necessary.
The reservoir is no longer in active use but, along with the nearby Lisvane reservoir, is valued as a beauty spot and nature haven.
The area has been designated a site of special scientific interest after rare wax caps mushrooms were discovered there.
Western Power Distribution (WPD) has long wanted to build on part of Llanishen reservoir, creating 300 homes, along with a wetlands area, education centre and a new sailing club to replace the one that currently uses the site.
However, local people, environmental organisations and politicians have opposed the plans, fearing they will ruin one of the valued areas of open green spaces in the city.
Now, they have stepped up their campaign after learning that WPD planned to drain three metes of water from the top of the reservoir - the maximum it can remove without permission from the Environment Agency.
Pipes and fencing were brought onto the site last Friday and it is understood draining work has been carried out over the last few days, pumping water into the Nant Fawr stream.
However, Environment Agency Wales insisted that other techniques and technologies could be used for the inspection other than draining.
It also warned the company not to pollute the Nant Fawr or harm wildlife.
"We have written to Western Power to emphasise that they do not need to drain down Llanishen Reservoir in order to carry out their inspections.
"Indeed, there are other techniques and technologies the company could use to undertake the required inspections," a spokeswoman said.
"The company has a legal requirement to carry out this work under the Reservoirs Act, but we have at no point dictated that this requires the draining of the reservoir."
Local campaigners said they fear once the initial draining is completed, the company will apply for a license to pump out the rest of the water.
They claim it may damage the reservoir's walls, impact on wildlife and cause silt from the reservoir to drain along the Nant Fawr into nearby Roath Park Lake.
Ewart Parkinson, who became Cardiff's first city planning officer in 1964 and was director of environment for South Glamorgan for 11 years, said: "It's so frustrating. It's been going on for nearly 10 years now.
"If the reservoir goes we will lose part of Cardiff's stamp - no other city has the green spaces we have and it's important that we do not lose them."
A spokesman for WPD said: "We have started the drain down of the reservoir following the report by an independent expert.
"This action enables us to meet our obligations in terms of the periodic statutory inspection of the reservoir."