Hogweed?

'The Forager Handbook' by Mike Irving says similar species are Wild Angelica, Wild Parsnip, and Giant Hogweed (Heracleum mantegazzianum)

Giant Hogweed is a dangerous plant as it is 'phototoxic', causing potentially severe burns and potentially permanent blindness

Here's an interesting link about the edible Hogweed in the UK http://www.gallowaywildfoods.com/?page_id=948

The link warns: 'Care should be taken when picking as chemicals in the sap can cause phytophotodermititus – especially in strong sunlight – gloves recommended for the thin-skinned' It also stresses that Hogweed stems start off purple and turn green but is NOT blotchy purple like giant hogweed