Ragwort is pretty but poses a threat to grazing livestock

Tony Sangwine said ragwort, with its bright yellow flowers, was one of the more colourful plants on verges, but had "a more sinister role".

He said it was one of the most frequent causes of plant poisoning of livestock.

He said traditional control methods are to use herbicide at an early stage, or to pull out the plants when in flower.

'Perennial problem'

Efforts would be targeted on infestations of the weed growing close to animal pasture, Mr Sangwine said.

"Ragwort can be a difficult plant to combat," he added.

"Each plant produces up to 150,000 seeds which can lie dormant in the soil for up to 20 years causing the plant to grow unexpectedly in many different places.

"The Highways Agency takes this perennial problem very seriously."

Lee Hackett, a spokesman for the British Horse Society said the group was "delighted" that the Highways Agency was taking the problem of Ragwort poisoning so seriously.