World Rugby chairman Bill Beaumont said on Tuesday the sport was about to enter an "exciting new era" with the unveiling of a new bi-annual international competition from 2026, comprising a top division of 12 teams from the Six Nations and the southern hemisphere's Rugby Championship.
Two further countries -- thought likely to be Japan and Fiji -- will be invited to make up the dozen with matches being played in July and November.
There wil be a second division also made up of a dozen teams with promotion and relegation coming into operation in 2030.
The new competition -- yet to be given an official name -- will be played in July and November, replacing the current summer and autumn international windows.
Reforms will also be made to the women's game, meaning there will be no overlap between international and club fixtures for the first time from 2026.
World Rugby also announced that the men's Rugby World Cup will enlarge from 20 nations to 24 for its next edition in Australia in 2027.
"Agreement on the men's and women's global calendars and their content is the most significant development in the sport since the game went professional," Beaumont said.
"(It is) a historic moment for our sport that sets us up collectively for success.
"We now look forward to an exciting new era commencing in 2026. An era that will bring certainty and opportunity for all.
"An era that will support the many, not the few, and an era that will supercharge the development of the sport beyond its traditional and often self-imposed boundaries.
"I would like to thank all my colleagues for their spirit of collaboration. We have achieved something special."
The sport's governing body said in a statement that adding four nations to the World Cup would "provide more qualification opportunities for more teams and regional competitions".
The move comes despite several mismatches in the pool stages of the World Cup in France, which culminates in Saturday's final between defending champions South Africa and New Zealand.
Among the one-sided encounters were Romania's 82-6 and 76-0 defeats by Ireland and South Africa respectively, while Namibia went down 71-3 to New Zealand and 96-0 to hosts France.
Only Fiji of the tier two nations reached the knockout stages, the highlight beating Australia for the first time in 69 years, but they also lost to Portugal.
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