In Britain, and the rest of Europe, a Group One race is a horse race of the highest calibre, as designated by the European Pattern Committee. Group One races include some of the most prestigious, valuable and historic races in Britain, over distances between 5 furlongs and 2 miles 4 furlongs, on Grade One racecourses, such as Ascot, Newmarket and York.
Some Group One races, such as the ‘Classic’ races – that is, the 1,000 Guineas, 2,000 Guineas, Oaks, Derby and St. Leger – are restricted to certain age groups and others, such as the Nassau Stakes and Sun Chariot Stakes, are restricted to a specific gender. However, generally speaking, horses of the same age and gender compete at level weights in Group One races, with weight-for-age and weight-for-sex allowances for three-year-olds competing against older horses and fillies and mares racing against colts and geldings, respectively.
Of course, Group One races can occasionally be downgraded; to maintain Group One status, over a three-year period, the average official rating of the first four horses home in the race in question must be 115, or more. From 2018, in Group One races, other than two-year-old races, in Britain, a horse must have achieved an official rating of 80 to be allowed to run in the first place.