Two Rugby World Cup winners have expressed their reservations about the change in the number of foreign-born players allowed to play in the Japan Rugby Top League.

When the 2018-2019 season kicks off on Aug. 31, teams will now be able to play five foreigners at any one time, at least three of whom must be eligible to play for Japan either now or in the future. Clubs will be limited to six such players in their 23-man match-day squad. But once players born overseas who have taken Japanese citizenship and who are eligible to play for Japan are added to the mix as well as one Asian passport holder (such as Malaysia and Yamaha Jubilo lock Duke Krishnan), then there are sides that could start with just four or five Japan-born players in their run-on XV.

The increase has been brought in to try to raise standards in the league in the hope it has a knock-on effect on the national team ahead of next year's World Cup in Japan.

But opponents argue that the long-term effects could be harmful to the development of local talent.

"I don't think it's a good thing for Japanese rugby," Toyota Verblitz coach Jake White told Kyodo News at Monday's official season launch.

White, who was in charge of South Africa when they became world champions in 2007, said the system in Japan needs to change so that "Japanese boys are picked to play in the Top League."

(Jake White and the Toyota Verblitz in 2017)

"There are a lot of boys right now in Japan in their second or third year of university who in France (for the 2023 Rugby World Cup) would be 26 or 27 years old, which for an international player is the peak of their career. But they won't make the World Cup because the system is wrong. Those types of boys should be drafted now. But they have to go through the (university) system, graduate and then (generally) spend two years as a sort of rookie as an understudy to a senior guy at their club."

"We are all guilty, I'm not pointing the finger at any club but I think you are diminishing the chances of boys who want to play for Japan and could play for Japan long term."

As a way of explanation White pointed out that South Africa hooker Malcolm Marx at 24 years old has already played 54 Super Rugby games for the Johannesburg-based Lions since 2014, while Verblitz and Japan flanker Kazuki Himeno (who, against the norm, was picked by White as captain straight out of Teikyo University) has had just one season of professional rugby despite being the same age as Marx.

Toshiba Brave Lupus captain Richard Kahui was part of the New Zealand side that won the World Cup in 2011.

"It depends on who you are asking and what you are trying to improve," he told Kyodo News of the changes.

(Richard Kahui, left, in 2017)

"If you are looking purely at Top League then I think the standard of rugby has to be better with more foreigners and it will be a better and more exciting competition."

"But what does it mean for Japan? I'm not sure. If you look around at a position such as (flyhalf) for example, you have Yu Tamura and the guy from Panasonic and Kosei Ono, but after that you look at every Top League team and most of them have a foreign 10. So how are they going to develop Japanese players? It's a real concern for me and Japanese rugby as you move forward."

The lack of local talent is also a concern in the forwards. Canon Eagles' five locks for the 2018-2019 season, for example, contain just one -- Shohei Yuzawa -- born in Japan.

"If the foreigners coming here commit to Japan (rugby) and commit their lives to Japan then I feel that is the scenario that will work out in the long run," said Kahui, who arrived at the Toshiba club in 2013.

The shortened Top League season kicks off on Aug. 31 with the final set for Dec. 15 to allow national team members plenty of time to prepare for next year's Super Rugby season and World Cup.