More than 90,000 shoppers have handed over£10 for the cotton polka-dot dress and, if the trend continues over the summer, it is likely to find a place in the record books.
Primark normally expects to make 20,000 to 30,000 sales of a hit item.
The line made its debut in the shops in January and has been repeated several times because of its immense popularity. Ordinarily, the value chain changes its product mix round quickly, with new items going into stores weekly.
The dress comes in blue, navy, chocolate and yellow. A new chiffon version is being introduced, costing£12.