The prime minister found himself in deep water with the people of Cornwall today over unguarded remarks about its ancient riverine boundary.
Campaigners are due to take to the roads and water this weekend to protest about possible constituency boundary changes that could erode the traditional dividing line between Cornwall and Devon, the river Tamar.
People on both sides of the border are angry that reforms going through parliament could lead to at least one "Devonwall" constituency, straddling Cornwall and Devon.
Cameron made the remarks as he prepared for a television interview with ITV's the West Country Tonight at the Conservative party conference in Birmingham. "It's the Tamar, not the Amazon, for heaven's sake," he said.
He went on to say he would not "put it like that in the interview proper", but the station broadcast the casual remarks.
Alison Seabeck, Labour MP for Plymouth Moor View, called for Cameron to explain his comments.
"This is a very contentious subject, and I think Mr Cameron owes the public an explanation," she said. "This is a very sensitive issue.
"This does little to detract from the opinion of some members of the public, who think politicians say one thing publicly, but think another privately."
Mike Chappell, secretary of the Celtic League's Kernow branch, said Cameron's remark mocked the importance of the Tamar and "typified London's attitude to Cornwall and the Devonwall constituency plans".
"He appears to be trivialising something which runs deep through the veins of the people who oppose this constituency merger.
"Cameron's comments have added fuel to our fire. The River Tamar is one of the oldest borders in Europe, and Westminster is completely out of touch with us in Cornwall, our MPs and our businesses."
Cameron said in the interview that it was important to have constituencies of the same size.
He won favour in Cornwall in August when his newborn daughter was given the middle name Endellion, after a Cornish village close to where the Camerons were holidaying.
