Japanese rugby Tuesday announced tightened eligibility rules in order to give local players more time on the pitch but which could have repercussions for naturalised internationals.
More than half of the players who started last season's final of the Japan Rugby League One championship, the game's top tier in the country, were born overseas.
Foreign-born players have also played a prominent role for Japan's national side, who stunned South Africa at the 2015 World Cup and reached the quarterfinals on home soil four years later.
From the 2026-27 season, Japan Rugby League One clubs will have to field at least eight players at the same time who were raised in the country.
Exceptions will be made for players who have more than 30 caps for Japan.
The league said the new rules were needed to encourage more Japan-born players to pursue professional rugby careers.
League chief operating officer Hajime Shoji said it was "a difficult balance".
"No players are being shut out of the system," he said.
"We are simply looking to reset the balance in favour a bit more towards Japanese players as League One is, after all, a Japanese competition."
Were the rule to come into force today, several current Japan internationals including centre Dylan Riley and lock Warner Dearns would not be classified as home players.
That means fewer eligible places in League One sides for Japan internationals born overseas, leaving them at risk of less playing time.
"It's important to acknowledge that clubs have supported the change," said Shoji.
"We are grateful for their support and the input they have had in the process."