World Rugby's controversial plan for a new nations championship has suffered a major blow with both the English and French clubs threatening to take legal action against the governing body, Telegraph Sport can disclose.
It is understood that Premiership Rugby and Ligue Nationale de Rugby are considering action because they regard the proposal for an annual 12-side tournament - first revealed by the Herald - to be "in clear breach of the San Francisco agreement", the accord struck by World Rugby in January 2017 to guarantee the structure of the global season to 2032.
Telegraph Sport has seen a copy of a letter sent by LNR chairman Paul Goze to World Rugby chairman Bill Beaumont expressing its frustration at a lack of consultation with the French Top 14 clubs over the project, also known as the "World League", and warning that legal action could be taken unless "major concerns" were addressed.
A Premiership source said on Thursday that the English clubs were "100 per cent" behind their French counterparts ahead of crunch talks in Dublin next Thursday and had been angered that they had not been invited to take part in the negotiations, which will be attended by the heads of all tier-one unions, plus Japan and Fiji, and the International Rugby Players organisation.
Sources said English clubs were "astonished" they were not part of the process to plan the future of world rugby having been led to believe that the San Francisco accord have provided certainty for the next 12 years.
The English clubs have also recently come to an agreement with the Rugby Football Union that front-line players such as Owen Farrell and Maro Itoje would be rested for summer tours in the year after World Cups for player welfare purposes.
"We quite sensibly have put in a break there for player welfare purposes," said one source. "And another principle of San Francisco was that we were meant to help the developing nations in the summer after the World Cup, with England going to Japan next year while the senior players are given time off to recover.
"This has got to be seen in a four-year World Cup cycle. World Cup, summer off, Lions tour, tour of some sorts to get the players ready for the World Cup a year before the World Cup. World Rugby say they are addressing player welfare but how do you rest the top players when a team is involved in competitive games across the year?"
European Champions Cup sources are also thought to be perplexed about the proposal, after World Rugby released a video on Wednesday confirming that the climax of the tournament was proposed to be played over five weekends in November and December. World Rugby regulations state the international window covers three Tests.
Another source said: "World Rugby are trying to impose a model without consulting us while trying to wreck the tournaments already in place here which are generating significant revenues for the game. It simply has no chance of going through."
Significantly, the publication of the details about the proposed tournament, which would introduce promotion and relegation to the Six Nations as part of the over-arching competition that would also take in the Rugby Championship and climaxing with play-offs in November, does not appear to have any positive impact on the RFU's position, which is said to be "at best tepid" to the proposal ahead of the talks next week.
"The RFU remains to be convinced that the world league proposals are a better option than we have today," said an RFU spokeswoman. "Player welfare is very important to us, and at the moment we have more questions than answers on a number of fronts." World Rugby declined to comment.