The Celtic League will adopt a more traditional format as of next season after representatives from Wales, Scotland and Ireland agreed to make changes.
The tournament - which previously saw the top four from two pools progress to a knock-out stage - will now be made up of just one league with each team playing each other home and away.
Welsh Rugby Union chief executive David Moffett believes the decision - at a meeting in Barry today - is a major breakthrough for the Celtic League.
He said: "The accord we have agreed for the Celtic League to be played on a proper home and away basis from next season is a huge step forward for the tournament.
"From a Welsh perspective, the announcement that our five new regional teams will be competing in a full and proper fixture list next season is great news.
"It means there will be enough quality rugby throughout the season to hone the senior players, develop the younger talent and give the fans plenty of meaningful action.
"It means there will be top-class, professional club rugby being played throughout the rugby World Cup, giving the teams greater continuity than in the past.
"It also means that the international coaches will be able to see their players all the way up to a few weeks before they traditionally leave on summer tours.
"As a group, the Celtic League Association has taken a huge step forward with this agreement. We have also ironed out all issues regarding finance for next season."
Bill Watson, chief executive of the Scottish Rugby Union, added: "We have sought for some time to see the Celtic League expanded into a season-long competition.
"The restructuring of the game in Wales has enabled the Celtic countries to achieve our objective in a hugely meaningful way.
"Our competition next season will deliver more meaningful games of rugby, raising the intensity of every game played. I know the three Scottish teams will welcome this very significant development."