World Rugby has approved a groundbreaking motion that will allow players switch national allegiance during their careers.
The new regulation, which will come into effect from 1 January 2022, had been lobbied for in particular by the Pacific Rugby Players Welfare group in recent years.
It means that players can represent a second team in Test rugby, provided they have a blood connection to the country, and have not been capped in the previous three years.
After the motion was passed by the World Rugby Council, president Bill Beaumont described it as a "landmark" day for the game, adding that there will be certain conditions to ensure the ruling is not abused.
"Approval of this landmark regulatory change is the culmination of detailed and widespread modelling and consultation across the game," he said in a statement.
"We have listened to our membership and players and sought to update the regulation recognising the modern professional rugby environment without compromising the integrity of the international game.
"Any player who wishes to transfer will need to have a close and credible link to their new union, namely birth right or parent or grandparent birth right while meeting strong criteria, including a 36-month stand down period.
"We believe that this is the fairest way to implement progressive change that puts players first while also having the potential to support a growing, increasingly competitive international men's and women’s game."
Pacific Island nations such as Tonga, Samoa and Fiji look set to benefit instantly from the law change, with former All Blacks Malakai Fekitoa and Charles Piutau becoming eligible for Tonga as of January, while other former New Zealand internationals Lima Sopoaga and Steven Luatua have expressed interest in representing Samoa.
International Rugby Players CEO Omar Hassanein said: "The proposal to change the rules around player eligibility is something that we have worked on over many years with our member associations.
"Many players across the world will now benefit from the chance to represent the country of their or their ancestors' birth, serving as a real boost to the competitiveness of emerging nations, which in turn, will benefit the game as a whole."
Follow Bulls v Munster (Saturday 5.45pm) and Leinster v Ulster (Saturday, 8pm) via our live blog on RTÉ.ie/sport and the RTÉ News app. Watch Connacht v Ospreys (Friday 7.35pm) and Leinster v Ulster live on RTÉ2 and the RTÉ Player or listen to the inter-pro derby on RTÉ Radio 1 Extra.