The Environment Agency has said that over 700t of floating pennywort has been pulled out from the Duke of Northumberland River in west London.
Approximately 730m of the river had become hidden beneath a 550m-long carpet of the weed, the agency said.
The pennywort weed can be removed from watercourses by hand, but the Environment Agency said this particular outbreak proved a bigger challenge.
For 10 working days, contractors used a Truxor amphibious machine with a mechanised front rake attachment to clear the watercourse of the invasive plant.
This work took place alongside a manual operative team equipped with drag rakes and scoop nets to collect and clear swathes of floating pennywort at varying depths of the river channel.
“Floating pennywort, a highly invasive species was brought to the UK from the Americas in the 1980s by the aquatic nursey trade,” the agency said.
“Although it has been banned for sale for nearly a decade, it’s now in our watercourses and impossible to remove entirely.”
Rachel Keen, an area Environment Agency operations manager, said: “This was important work to do in the run-up to Christmas to maintain the health of the river channel and the fish and wildlife that depend on it.
“Clearing out this vast amount of weed not only removes an invasive species that harms the local environment, but one which can also cause obstruction to river flows.
“Our field teams routinely carry out pennywort-removal in our London rivers, and this summer’s extremely hot weather really boosted its growth.
“We’d encourage the public to be on the lookout for pennywort, hogweed and other invasive species, and report possible sightings to our incident hotline on 0800 80 70 60.”
