He might have been any other shopper, popping into the supermarket after a busy day in the office to pick up a few bits for dinner.
But the man queueing up at the checkout with a basket full of fresh fish and vegetables for a family meal had in fact just dealt with a resignation from one of his Cabinet ministers.
The Prime Minister made the impromptu visit to a branch of Morrisons on his way home to Oxfordshire after spending the day in Plymouth, Devon, meeting Royal Marines.
He spent about 15 minutes in the Plymstock store, causing other customers to do a double take as they saw who else was browsing the aisles.
Mr Cameron selected items for his dinner including sea bass from the fish counter, new potatoes, lemons and other fruit.
One shopper who was in the shop during the unannounced visit said: "He went in the queue and paid for it himself. There was quite a conservative reaction, with a small c. People noticed him, but no-one mobbed him.
"He had the most people chatting to him as he was in the queue - it took him about five minutes to get served."
Shoppers seemed so nonplussed by the visit that the man in front of the Prime Minister was playing with his phone as he waited to pay for his goods, seemingly oblivious to the VIP in the queue.
Mike Cliff, general manager of the Plymstock store, said: "We were surprised and delighted to see the Prime Minister popping into Morrisons to pick up his weekend groceries."
Mr Cameron visited Stonehouse Barracks in Plymouth, headquarters of the Royal Marines, before he made his unscheduled detour to the supermarket.