Ragwort

This is a plant, officially classified as an "injurious weed" by the Department for Food and Rural Affairs in the UK as an injurious weed. Ragwort is poisonous to horses, ponies, donkeys and other livestock, and causes liver damage, which can have potentially fatal consequences. Under the Weeds Act 1959, the Secretary of State may serve an enforcement notice on the occupier of land on which injurious weeds are growing, requiring the occupier to take action to prevent the spread of injurious weeds.
Defra have published a webpage, giving information.

This does not just applies to land on which animals are actually grazing - it applies anywhere. Ragwort grows up to several feet tall, flowers with pretty yellow flowers, and releases its seeds in puffballs that float on the wind, and can be carried for miles.

If you want to know what it looks like, Defra have published an information leaflet which gives an accurate description. Note: this is a PDF-file, for which you require Acrobat Reader on your computer; if you haven't got the Reader, you can download a free copy off the Acrobat website.

Ragwort should be eliminated where found, ideally at this time of the year, before the plants start to flower. They should be pulled out, root and all, and destroyed by burning. It is a very pretty plant - but lethal to lifestock. If you find it in your garden, get rid of it. Remember, if you don't, there are circumstance where you could conceivably be forced by law to do so.