Lewis will lead the Welsh delegation at a showdown meeting in Heathrow Airport on Friday at which the whole future of the professional club game in three Celtic nations will be on the line.
When the countries last met in May it ended in acrimony with the Scots and Irish throwing Wales out of the tournament and agreeing to play among themselves next season. They were unhappy at the way Wales had successfully negotiated a deal with the RFU to take part in an Anglo-Welsh Cup competition over the next four years.
The argument from the Scots and Irish was that Wales had not been "open and honest" with them about their dealings with England, that by taking part in the tournament Wales were undermining the integrity of the Celtic League and that the five dates required simply could not be fitted into the calendar.
But in a "clear-the-air" memo to the Scottish and Irish Unions, as well as the Celtic League broadcast partners, Lewis has moved to set the record straight on a number of those issues ahead of Friday's meeting. And the WRU Chief Executive remains convinced there is enough flexibility within the season to ensure that the Welsh regions neither compromise the integrity of the Celtic League, nor put unfair pressure on their rivals through fixture compensation.
"The first principle is that the WRU and its four regions are totally committed to the Celtic League. It is our bread and butter, our main ranking tournament for European competition," said Lewis. "Ever since the WRU moved to bring the Scottish teams into the Welsh/Scottish League, and then backed the addition of the Irish to make it a proper Celtic League, we have given our utmost support to the tournament.
"We believe that the format of the last two seasons, where there are 20 fixtures on a home and away basis, is near perfect to meet the needs of everyone in the three nations.
"However, in the early part of this year we were faced with the prospect of being dragged into a 'Rainbow Cup' involving South African provinces, and then excluded from meetings that tried to get four Italian teams into a re-shaped Celtic League played on a home or away basis.
"Against such a background of uncertainty we had to find ways of increasing and improving the fixture lists for our regions - for the good of their players and their fans. That's why, having initially rejected an offer from the RFU and their Premier Division clubs, to play in an Anglo-Welsh Cup we were forced to re-open negotiations with them.
"What has come out of those negotiations is a four-year deal to play in a cross-border cup competition with England's top dozen clubs that will require five weekends in the season - three for Pool matches and two for semi-finals and a final.
"The Scots and the Irish say they cannot find a way to accommodate those five weekends. We will be presenting them with further plans that should allow them to find a solution.
"We simply have to find a way through this or we are all going to suffer. Our broadcasters will leave us, sponsors will walk away and the fans simply won't have the same level of rugby to watch.
"We believe we have found more than one way for the Celtic league season to support our plan to play in the Anglo-Welsh Cup. If the Scots and Irish turn down those plans then they will simply be cutting their own throats along with ours."
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