A simple square scribbled on a yellowed piece of paper believed to have been drawn by Renaissance genius Michelangelo will go on sale in April in New York, auction house Christie's announced on Friday.

The company's experts were examining a drawing by another artist of the same period for a forthcoming sale when they saw, stuck to the back of the frame, "this little piece of paper with a drawing and a letter," Stijn Alsteens, international head of the Old Master Drawings department at Christie's, told AFP.