An invasive garden pond plant is threatening waterways in the Broads, the Environment Agency has said.
The American Floating Pennywort has taken over waterways
The American Floating Pennywort, which grows at up to 10in (25cm) a day, clogs up waterways and threatens to smother native British plants, the agency said.
The Broads Authority said the plant had grown over 10,000 sq ft of the River Waveney at Diss in the last few months.
Broads users have urged to inform the authority if they spot any large spanning weeds blocking waterways.
An agency spokesman said it was believed that Floating Pennywort had been dumped into wild waterways from people's garden ponds.
If it colonises the Broads it could compromise navigation and transport, the Broads Authority said.
Chris Adams, ecological appraisal officer at the Environment Agency, said: "The weed grows so thickly it can look like solid ground.
"This could put people and animals in danger if they don't realise there is deep water underneath.
"There is also a flood risk involved when this weed is dense in a watercourse.
"In major floods it could wash off into flood plains dispersing into and blocking smaller water bodies making it even more difficult to remove."