The event at Simonsbath on the border with North Devon to mark 70 years since the landmark 1949 National Parks and Access to the Countryside Act that gave rise to UK National Parks was passed. Prince Charles and HRH Camilla will join 500 picnickers at Simonsbath's riverside meadows to mark the anniversary. They will meet Exmoor National Park staff, local producers and craftsman, while enjoying a showcase of some of the area's culture, history and local breeds. To help mark the occasion, Their Royal Highnesses will be joined by Niall Hobhouse (grandson of Arthur Hobhouse who led the 1949 Act) and representatives from the UK National Park family. Sarah Bryan, chief executive of Exmoor National Park Authority, who are hosting the event, said: "We're thrilled to be welcoming Their Royal Highnesses to Exmoor and deeply honoured to be hosting this celebration. "National parks offer so much to the country and simply wouldn't exist were it not for the foresight and determination of those who fought for their establishment. "We have been amazed and encouraged at the strong demand for tickets from those wishing to help mark this special occasion. We regret anyone without a ticket will be unable to join us due to the limited capacity of the site." Rachel Thomas, chair of the Exmoor Society, which have helped organise the event along with the Exmoor Hill Farming Network (EHFN), said: "As an independent charity, our aim is to protect and promote Exmoor's national park status. "The Big Picnic at Simonsbath will celebrate all that makes Exmoor unique - not only its beauty, wildlife and heritage - but also upland farming livelihoods, produce from the land and the connection with nature so necessary to people's wellbeing. "So we are more than delighted that Their Royal Highnesses will be coming, recognising the national and international importance of the UK's national parks for all."