Teams in the Grand Tours and several other key races will have one less rider from next season in order to improve safety and enhance the spectacle for fans.
Teams in the Tour de France, the Giro d'Italia and the Spanish Vuelta will have eight riders instead of nine and several key races will feature seven-man teams instead of eight-man outfits.
The Amaury Sport Organisation (ASO), organisers of the Tour and the Vuelta, RCS, who run the Giro, and Flanders Classics between them own 17 of the 37 World Tour (elite) races, including the top one-day classics.
"Following the General Assembly of the International Association of Cycling Race Organisers (AIOCC), RCS Sport, Flanders Classics and ASO have taken the decision to reduce the number of riders per team at the start of their races," a statement said.
"This decision has a two-pronged objective: the first being to improve the safety conditions for riders with a smaller peloton on roads equipped with more and more street furniture.
"The second, which is a consequence of the first, is to make it more difficult to dominate a race as well as to enhance event conditions to offer better racing for cycling fans."
The statement added that the reduction would come into effect for the 2017 season, while the number of teams would remain the same.