The Prince, who will be 2-years-old on July 22, was wearing a smocked set of red shorts and embroidered shirt by the British designer Rachel Riley, a favourite choice of his mother Kate Middleton for her children's clothes.
This morning the company's website had sold out of the 85 pounds-outfit in 12 month and 18 month sizes, The Telegraph reported.
For the second time in a matter of weeks, Prince George wore an almost exact replica of an outfit worn by his father the Duke of Cambridge on a similar occasion.
After the Trooping the Colour ceremony last month, George appeared on the balcony of Buckingham Palace wearing a near-identical sky blue outfit to the one worn by Prince William on his own balcony debut in 1984.
The red 100 per cent cotton outfit worn by Prince George at Sunday's christening is described on the Rachel Riley website as "hand smocked across the chest with a vintage inspired design". The shorts can be buttoned onto the shirt for a formal look.