EXPERTS fear the River Thames could be completely destroyed - by an invasive alien plant.

Floating Pennywort, which can grow up to 20cm a day, has been spreading across the south east of England, and has now entered the central section of the river.

The Triffid-like plague could eventually swamp the entire river if left unchecked, killing all the native species of plant and fish.

The weed blankets the river surface, blocking the waterway to boats and anglers and suffocating plant life on the river bed.

Environmental specialist Professor Alastair Driver, said: "Floating Pennywort has been spreading across the south east, and there has been a population in the lower Thames for some time.

"But in July, it was first found in the central Thames, where it has entered from the river Kennet and its tributaries in Reading.

"It is now in many locations, at least between Reading and Henley, across ten miles of river.

"This has several potential impacts, because it grows phenomenally quickly - around 20cm a day.

"It could cause problems for anglers too, because they wouldn't be able to physically get their tackle into the water.

"Being a blanket, it shades out the water beneath it, so plants can't grow, and it could change the ecology of the river."

This summer’s low river flows and warm temperatures have contributed to its rapid spread.

Professor Driver said the spread is "concerning" and urged the Environment Agency to remove the weed.

He added: "We are concerned about it and the basic message is let's get it completely removed and stop it coming into the Thames.

"Every last fragment needs removing, and they need additional resources from the government to let them do it quickly.

"If it's left until the rainfall and flooding, it's too late."

The Environment Agency said they are working hard to remove the weed, and have an action plan in place to remove the plant before it spreads.

A spokesman said: "We have identified the presence of the non-native invasive plant Floating Pennywort at multiple locations in the River Thames.

"Landowners have a legal responsibility to control the growth and spread of certain invasive species on their property including rivers.

"However, in this case the Environment Agency is using its permissive powers to quickly control this new infestation to prevent further spread, which will be more difficult and more expensive to deal with if not tackled early.

"In the spring, we will reassess the extent of Floating Pennywort and initiate herbicide spraying or undertake careful removal by hand or machine at key locations."