Maldon's 200-year-old canal could be facing a major environmental threat if weed choking its waters is not removed, experts have warned.

Despite recent work to cut back the fast-growing weed - believed to be a variety of pennywort - it is again spreading quickly up the 14-mile Chelmer and Blackwater canal.

The rivers Chelmer and the Can in Chelmsford are also badly affected.

The weed, which grows about a foot above the water and trails a matted carpet of roots, is already in evidence at Beeleigh.

River conservationists have expressed alarm at the situation, while the weed is already causing problems for boats.

John Marriage, of the Friends of the Chelmer and Blackwater Canal, said that waterlilies, a feature of the beauty spot at Beeleigh, are being destroyed by the weed, while its effect on fish and the local swan population is still unknown.

Mr Marriage said: "This weed is choking our canal. Once firmly established it reaches from bank to bank. Below water level thick matted roots create an impenetrable carpet."

The weed first appeared on the upper Chelmer and Can about three years ago, he said.

"There is a real danger that the plant will spread along the entire canal and in the opinion of The Friends it needs to be destroyed before it can become well established.

"We are very concerned that the weed has got as far as Beeleigh. We are now worried that it will invade the Long Pond. This stretch of water could become unusable for both boats and anglers."

Already it is becoming impossible for canoeists to use stretches of the rivers and canal, he said. Even the pleasure barge Victoria - and the Susan, the last working vessel on the canal, were finding it hard to make progress.

Steve Worrall, Environment Agency engineer for Essex, said that national research is being carried out into ways to control the invasive weed.

Twice yearly cutting and clearing work by the agency had failed to control the weed, which grows in clumps, while any control method has to be sensitive to the riverside habitat as a whole.

The weed, which resembles a buttercup, is a foreign species, believed to have been introduced to the river from an ornamental fish pond.

"It is a very difficult problem, it just grows back," Mr Worrall said.

"Our biggest concern is the impact on the river environment. The weed shuts out sunlight and stops aquatic plants growing and affects other forms of water life.

"This is a national problem for rivers and at the moment we haven't got any real answers."

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