Ben Jones, of St Ives firm Jones Boatyard, said the Great Ouse and the River Cam network were at risk from the weed and native species were in danger. He warned that and that floating islands of the weed were already appearing in the waterways, increasing the risk of flooding as well as making the river unnavigable. The Environment Agency, which has been controlling the weed, said that in one location a mat of weed weighing 1.7 tonnes had been removed from the river. The weed, which can grow at up to 20cm a day, is native to the Americas and was introduced into the UK in the 1980s for use in ponds and fish tanks but escaped into the wild a few years later and was banned from sale. Mr Jones said: It is very damaging to fish stocks, birdlife and indigenous plants as well as increasing the risk of flooding in addition to making thee river unnavigable. It is in real danger of overrunning this river system in the next 12 months. He said: We particularly worry that if the pennywort were to enter the Old West section of the river, where there is no natural flow, it would be much more difficult to eradicate. A spokesman for the agency said: We currently have contractors undertaking removal work on the River Great Ouse, Bedford to St Ives,in line with our strategy and action plan. We also work with various volunteer groups across the area to assist in tackling floating pennywort. If we become aware of growth potentially blocking navigation we issue navigation notices to warn river users.. The spokesman said: We issued a navigation notice to boat owners identifying they can help to reduce the spread of this species by following the Check, Clean, Dry principle and by visually checking their craft regularly and removing any visible weed and by placing it well away from any river or other water body. If people spot floating pennywort it is important that they dont contribute to its spread. If boats are driven through clumps of the plant it can break off and regrow elsewhere. The spokesman added: Rather than try to remove the plant it is important to report any sightings of floating pennywort to the Environment Agency via our free Plant Tracker App or by calling our national customer contact centre on 03708 506506.