An invasive pond plant is threatening to suffocate native plants and harm wildlife in a wetland nature reserve in Dorset, the National Trust has warned.
A survey of the freshwater lake Little Sea at Studland and Godlingston Heath in Purbeck showed a rise in Crassula.
It was introduced as an ornamental pond plant in the late 1970s but has since been dumped or escaped from gardens.
The trust has called it a "swamp monster" and has asked the public to help stop it from spreading.
Crassula helmsii is a lime green plant with small white flowers, also known as New Zealand pygmy weed or Australian swamp stonecrop.
It covers open water with a thick green blanket, depriving birds and animals of habitat and threatening fish and invertebrates by removing oxygen from the water.
Although illegal to sell in the UK, Crassula is still present in many garden ponds and the trust said it had evidence of gardeners sometimes deliberately dumping in their bid to get rid of it.
It believes the plant was transferred from another body of water to Little Sea by a bird.
Just a tiny fragment can quickly reproduce many times over and the invader has no natural enemies in Britain to control its spread.
Once established in the wild, it is almost impossible to eradicate, Simon Ford, countryside advisor at the National Trust, said.
"This is probably the most difficult of all the invasive species to deal with," he explained.
"Whatever you do, whether it's using herbicides, physical removal or covering it over with black plastic, it still seems to come back.
"Obviously, the first thing to do is to try and stop the thing getting out into the environment in the first place.
"If you find Crassula taking over your garden pond and want to get rid of it, please don't dump it.
"Put it in a black plastic bag, seal it and leave it for at least six months. Burn it once it is completely dead and dry."
The lake and wetlands at Studland and Godlingston Heath is an important habitat for birds and dragonflies, and rare plants such as sphagnum mosses, sundews and St John's wort. They are all directly threatened by Crassula, Mr Ford said.