The letter underlines the governing body's stance on a range of issues in the row and why it will not sanction an Anglo-Welsh league.
It also claims the new deal intends to ensure "Welsh player development and the retention of Welsh players is properly recognised".
The Scrum V debate: The future of Wales' rugby regions
The letter confirms that the regions - Scarlets, Ospreys, Cardiff Blues and Newport Gwent Dragons - intend to break away from the WRU after 30 June, 2014 if no agreement is reached.
The three-page letter makes 50 bullet points to outline the current state of play between the two sides.
It says the WRU "will not agree to any Welsh club or regional organisation playing in competitions which have not been agreed by the International Rugby Board" and adds that "an Anglo-Welsh league or a non-sanctioned European tournament are not viable in the best interests of Welsh rugby or the games across Europe".
The Union also reiterates it is contracted to the Heineken Cup and the Pro12 and will not "countenance leaving Scotland, Ireland and Italy isolated from meaningful tournaments".
"Apart from the moral imperative such a move would inevitably lead to a spiral of decline for their national teams and consequently the demise of the Six Nations into a meaningless and unattractive sporting competition," it added.
The letter does acknowledge the attraction of an Anglo-Welsh league but says it is not realistic because of current contractual agreements.
It also addresses the financial deal at the heart of the participation agreement and claims the deal rejected by the regions would have seen them get £7.6m directly from the WRU in 2019.
But the WRU does admit Wales will never be able to compete with the "powerful commercial income streams" in England or France.
It says: "The growing salary levels for players in France and England cannot be matched by the constant distribution of cash from within WRU resources.
"Instead we have to encourage our regions to grow their own businesses by improving their structures and commercial synergies to build support and stability."
The letter confirms that the issues of central contracts was discussed with the regions in August 2012 and that in 2013 the governing body offered £1m to sign "six marquee players" nominated by the regions but that they did not respond to the offer.
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