World Rugby doubled its international window for the autumn tests on Thursday, despite the opposition of the English and French Leagues.
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The decision by the World Rugby Council on Thursday to approve a temporary 2020 calendar following the coronavirus shutdown means that international teams can play six autumn Tests instead of the normal three.
That allows international tournaments postponed because of the pandemic to catch up, but potentially at the expense of club fixtures.
The LNR, which runs French club rugby, said it has taken a first step towards legal proceedings by issuing a formal notice to World Rugby, accusing it of modifying the international calendar "without any real consultation with the professional leagues and in particular the LNR."
Under the World Rugby plan, the Six Nations, put on hold in March with one round of games plus the Ireland-Italy match still to play, will be completed on 24 and 31 October, with a rest weekend after that, followed by four consecutive rounds of international matches with Fiji and Japan as likely guests.
The southern hemisphere Rugby Championship will take place over a reduced six-week period between 7 November and the second weekend in December.
Southern hemisphere governing body Sanzaar (South Africa, New Zealand, Australia and Argentina Rugby) has asked New Zealand to host the tournament.
The English Premiership is due to resume next month and the knockout stages of the European club competitions in September.
The Premiership final is taking place on 24 October, the same day the Six Nations resumes.
World Rugby said in a statement that approval for the temporary calendar came after "extensive and productive dialogue" between the sport's major stakeholders, balancing the interests of the international game, the professional club game and player welfare.
A fierce debate has raged over potential schedules, with the Premiership and French league as well as players' unions concerned over a possible fixture pile-up.
"Losing so many players for five or six weeks plus a weekend off, which is still a long time coming, is a lot," said Didier Lacroix, president of 2019 French champions Toulouse, who provide the backbone of the french team, in response to the announcement.
"We want to keep supporting the French national team but we are asking for a balanced effort," he said, calling for "the integrity of the players to be put back at the centre of the debate".
Fabien Galthie, the France coach, was happy at the prospect of six games.
"The more you play together, the more you progress," he told Sud-Ouest newspaper on Wednesday.
One of his predecessors Bernard Laporte, now the World Rugby vice-chairperson said: "This has been no small effort and there have been some robust conversations, but the position approved by council today represents the best interests of the global game and I look forward to seeing the world's best players doing what they do best back on the international stage."
World Rugby chairperson Bill Beaumont added: "Throughout this process, all parties have sought to deliver the best possible outcome to support the interests of international and club rugby and the players.
"Agreement and approval of this schedule is exciting for players and fans and an important step in supporting our unions in mitigating financial impact and optimising the sport's return from the pandemic in an equitable way."