Prickley Lettuce Lactuca serriola

It is very easily identified by the row of prickles up the underside of the midrib of the leaf. It can grow quite tall and has small yellow flowers open in the morning. It is often known as Compass plant because the upper stem leaves fully exposed to the sun are held vertically in the north south plane. Closely related to the lettuce hence probably not poisonous but very likely to taste very bitter.