The title of champion, or leading, sire of racehorses in Great Britain and Ireland is awarded to the stallion whose offspring have won the most prize money in Britain and Ireland during the flat racing season. The current champion is Frankel, who replaced his sire Galileo as the leading sire in 2021 after Galileo had won the title twelve times.
Unlike the similar title for leading sire in North America, the stallion in question does not need to have resided in Great Britain or Ireland during his stud career, although the vast majority have done so. Northern Dancer is the most notable example of a North American-based stallion who won this title. The Northern Dancer sire line has dominated the list for the last several decades, mostly through his son Sadler's Wells (14 titles) and grandson Galileo.