For the 15th June 2009 Hansard, the official record of the UK parliament contains the following information Tim Farron, who is the Liberal Democrat Environment spokesman , asked the following question. The question is about another plant but the answer provides us with an official statement on ragwort as well.
To ask the Secretary of State for Environment, Food and Rural Affairs if he will bring forward proposals to classify hogweed as a notifiable weed.
The question is odd in itself, and shows a poor understanding of botany and the issues around plant toxicology in relation to other plants not just Ragwort. Hogweed, sometimes called "Common Hogweed" to distinguish it from the non-native species is not just harmless but actually edible. It is Giant Hogweed an invader from the Caucasus that is a problem plant.
However the important thing is the answer which was given by Huw Irranca-Davies, a parliamentary Undersecretary of State for DEFRA, which is the government department responsible for this issue, the salient part of which is gven below.
We have no plans to make either the native hogweed or the invasive non-native giant hogweed "notifiable.weeds" The Weeds Act 1959 is a consolidation of earlier legislation concerned with commercial food production. The five weeds covered by the 1959 Act are common ragwort, spear thistle, creeping or field thistle, curled dock and broad-leaved dock. Common ragwort is poisonous to animals, in particular horses. The others, if allowed to spread, compete with and suppress grass and crop plants. The five weeds covered by the 1959 Act are not "notifiable" as such and there is no obligation to report their presence to anyone.