Llyn Brianne was completed in 1972 and supplies drinking water to a large part of south Wales.
The reservoir is up to 272 ft (83m) deep and holds more than 13 billion gallons (60 billion litres) of water.
A statement from Welsh Water, the owner of the site, said it "strictly forbids" water-based activities at the reservoir and would take "appropriate action" against anyone in breach of this restriction.
"Reservoirs can be dangerous for various reasons and those involved in water sports in inappropriate locations, such as at Llyn Brianne, put themselves and others at unnecessary risk," said a spokesman.
Richard Harvey from the Welsh Canoeing Association said he could not comment on the group involved, the locality or the legality of their activity but he did say he was aware the slipway had been used by kayakers for about 20 years.
"We encourage people to paddle responsibly and in the realm of the law," he said.
"It's very important that kayaking is done at no risk to other people or the environment and that a detailed risk assessment is made before taking to the water.
"Enthusiasts also have to be aware of the law and the possible legal restrictions relating to a particular stretch of water.
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