Network Rail has lost an appeal over damages awarded to two homeowners after Japanese knotweed encroached on their land in Bridgend county.
Stephen Williams and Robin Waistell, who own two adjoining bungalows, won a claim against Network Rail which owns the land behind their homes in Maesteg.
The court ruled in favour of the pair because the plant's underground roots had extended under their houses.
The decision could have an impact for landowners across Wales and England.
Announcing the decision, Master of the Rolls Sir Terence Etherton said: "Japanese knotweed, and its roots and rhizomes, does not merely carry the risk of future physical damage to buildings, structures and installations on the land.
"Its presence imposes an immediate burden on landowners who face an increased difficulty in their ability to develop, and in the cost of developing, their land, should they wish to do so, because of the difficulties and expense of eradicating Japanese knotweed from affected land."
He added that Japanese knotweed "can fairly be described as a natural hazard which affects landowners' ability fully to use and enjoy their property and, in doing so, interferes with the land's amenity value".
However, the judge said the homeowners would not be entitled to damages because the knotweed had lowered the value of their properties.