royal warrant 
noun
A warrant issued by the sovereign, especially one authorizing a company to display the royal arms, indicating that goods or services are supplied to the sovereign or to a member of the royal family.The factory secured its first royal warrant in 1775 with supplies to the king, to be followed with a dessert service for George, Prince of Wales, in 1784....
- ‘People are getting bolder and bolder,’ confirms Pippa Dougherty, spokeswoman for Hamilton & Inches, the royal warrant jeweller.
- Indeed, the pencil, with its cracked green glaze and white end, is the UK's best-selling premium pencil and boasts a royal warrant.