London - England's Rugby Players' Association hit out on Monday at what it said was a "detrimental" plan to extend the top-flight Premiership season by a month, saying it was not "viable" and would put international players especially under "incredible strain".
There have long been concerns about the toll an increasingly physical and fast-paced professional game is taking on players, with worries about concussion-related injuries a particular issue.
At the same time there has been an attempt to bring greater harmony between club and international fixtures worldwide, with rugby union traditionally a winter sport in both the northern and southern hemispheres.
In March, World Rugby, the sport's governing body, unveiled plans for a new global calendar for 2020 onwards which will include shifting the June Test window back to July to allow the southern hemisphere Super Rugby season to run uninterrupted.
But that threw up the possibility that, in effect, leading European-based players could be left with an 11-month season.
In a strongly-worded statement, the RPA said that, while disappointed to learn of league organisers Premiership Rugby's plans via a press release in March, they had invited them to present their proposals in more detail to the RPA Players' Board on Wednesday, May 10.
"However, after due consideration, the Board unanimously rejected these proposals in their current form," the RPA statement said.
"Extending an already arduous season from nine months to ten has serious implications for players, given the potential increase to the game, training and psychological loads they face."
Saying it had fought hard for the introduction of a mandatory five-week off-season break in the standard Premiership contract, the RFA insisted players also needed an "extensive" pre-season period to get ready for the demands of a league campaign.
The RPA said Test stars, who already face a gruelling schedule, would be particularly badly hit by the proposed changes.
"Perhaps most worryingly is the incredible strain these proposals would place on international players.
"If the Premiership season retains its current start date, the addition of a July tour schedule will lead to an 11-month season for these players.
"This cannot be avoided unless these players start their domestic season later, which brings into question the need for the season extension.
"Whilst we also acknowledge that the reduction of overlaps between international and domestic rugby is desirable, we do not believe the current proposals are viable."