This 2m tall plant was conveniently growing next to Lactuca serriola (Prickly Lettuce) so the difference between the two was obvious. On the plants we saw, L. virosa has distinctly lobed and wavy edged leaves while L. serriola's leaves were completely without lobes or any waviness. The leaves of L. virosa had a maroon tinge to them absent in L. serriola.

The flowers don't often open as in L. serriola so this flower head, taken on a very hot day when they would surely have opened if so inclined, is probably as good as you'll find in the UK.