Governing body Celtic Rugby is expected to confirm next week that South African teams Southern Kings and Cheetahs will be invited into the Pro12 competition.
Board members are set to meet in Dublin on Tuesday, 18 July to discuss the tournament’s immediate future.
The South African franchises, who play their final Super Rugby game on Friday, would join 12 teams from Wales, Italy, Ireland and Scotland from September.
The revamp could see the league divided into two conferences of seven teams.
If agreed, that would see teams play their conference rivals home and away, but they are expected to also play against teams in the other conference at least once.
It is understood clubs are eager to safeguard as many derby games in the season as possible.
Kings and Cheetahs bow out of Super Rugby
The Kings and Cheetahs are able “to explore other international opportunities” as they are poised to leave Super Rugby, the southern hemisphere’s premier club competition between clubs from South Africa, Australia, New Zealand, Argentina and Japan.
By coincidence, the teams play each other in their final game in the competition on Friday at the Nelson Mandela Bay Stadium in Port Elizabeth.
Super Rugby will be reduced from 18 to 15 teams next season, with an Australian side also poised to be cut.
Despite being set to play in a European league, the two teams are keen to remain based in South Africa rather than the UK.
There has been a suggestion they could play some of their home matches in the United Kingdom, with Saracens’ Allianz Arena mooted as a potential venue.
This would be the first major change to the Pro Rugby competition since the introduction of Italian sides in 2010.
Welsh and Scottish sides first combined in a league in 1999, before an agreement with the Irish Rugby Football Union saw the Celtic League formed in 2001 with 15 teams.
This latest development in the Pro12 could also open the door to North American participation for the 2018-19 season, as the competition explores expansion options to generate more revenue and tap into new markets.
There have also been concerns over the future participation of Italy’s two clubs, with Pro12 managing director Martin Anayi meeting with Italian rugby bosses this week in Rome.
Celtic Rugby was contacted by BBC Wales Sport, but declined to comment.