An invasive aquatic plant brought to the UK to decorate aquariums poses a serious threat to lakes and rivers, the Royal Horticultural Society has said.
New Zealand pygmyweed can form a thick dense mat across a body of water. It grows rapidly and can starve native species of oxygen and sunlight.
Scientists say it has crowded out nine native species in Derwentwater in the Lake District and is threatening wildlife in the New Forest.
The Outdoor Swimming Society has urged wild swimmers to avoid transferring fragments of the plant from Derwentwater to other lakes. Pieces a few millimetres long can grow into large plants.
Guy Barter, chief horticulturist for the Royal Horticultural Society, said: “It forms very dense mats for depths of up to three