The Pro14 Rainbow Cup will be split into two tournaments after South African sides were denied travel to Europe for the competition.
The new set-up was meant to feature the existing 12 Pro14 clubs in addition to the Bulls, Sharks, Stormers and Lions.
It was designed to act as a precursor to the permanent involvement of the South African sides from 2021-22.
Instead due to Covid-19, two separate tournaments will be held, one in Europe and the other in South Africa.
It means South Africa-based players in contention to face the British and Irish Lions in the summer will not have cross-border competition beforehand.
Leinster were last month crowned Pro14 champions in a curtailed 2020-21 campaign after a decision was made in December 2020 to shorten the season to create the new Rainbow Cup.
It was viewed as a way to generate revenue for the game ahead of the new South African sides joining the league next season and provide the Springboks players with competitive action ahead of the British and Irish Lions' arrival.
The new tournament was scheduled to start on 23 April and continue through to a final on 19 June.
The Bulls, Sharks, Stormers and Lions were set to stay in a bubble for the second half of the tournament and fly to Europe for games.
This has been scuppered after Pro14 bosses admitted no formal approvals were in place to allow the South African teams to enter the United Kingdom and Republic of Ireland because of Covid-19 concerns.
They have now been told they cannot travel because South Africa remains on the UK's red list of countries.
The red list is the government's list of 40 countries from which there are strict restrictions on travel to and, therefore, via England.
Northern Ireland has similar rules in place, while Scotland's are stricter and Wales does not currently have international flights arriving, while rules in the Republic of Ireland cover Munster, Connacht and Leinster.
The first three rounds of the Rainbow Cup will continue as scheduled for teams from Wales, Scotland, Ireland and Italy.
The four Irish provinces are playing each other over these opening weekends, as are the quartet of Welsh regions, while Scottish and Italian sides compete against each other.
Rounds four to six were scheduled to see the four South African franchises play their away games in Europe, but that has now been stopped after they were denied permission to travel.
The organisers say all options were explored, with 12 venues across UK, Ireland and Europe considered as base camps for the South African teams to operate from or to use as a quarantine destination.
SA Rugby also explored another four separate locations, with destinations in the Middle East also considered as potential hosts for fixtures.
The decision has been made to run dual tournaments with no cross-hemisphere fixtures.
The 'northern' Guinness Pro14 Rainbow Cup will still take place on the dates previously published, with the 12 teams from Wales, Scotland, Ireland and Italy.
The games in Europe intended to involve South African teams in rounds four to six have been removed and kick-off times may be modified, with some byes possible.
The 'southern' tournament will be called Rainbow Cup SA involving the Bulls, Sharks, Stormers and Lions, but they might not play on the opening weekend.
"A staggering volume of work has been undertaken to provide a number of proposals and options to accommodate this," said Pro14 chief executive Martin Anayi.
"All as we navigated the challenges of the second and third waves of Covid-19 as well as the South African variant, which constantly changed the landscape we were operating in.
"Among our unions, our staff and SA Rugby there is no more that could have been asked in terms of designing plans that were medically sound.
"However, there has been no perfect solution found in time to allow for South African teams' entry into our territories.
"Whilst the outcome is clearly different from what we had intended, our relationship and partnership with SA Rugby has been greatly strengthened and enhanced by this experience.
"We are looking forward to the two Rainbow Cup competitions and in due course sharing our intentions about our future partnership that will be boosted by the experiences and project planning involved to this point ahead of the 2021-22 season."
'Time ran out'
South African-expanded involvement will still go ahead from next season if travel restrictions allow. Two other sides, Cheetahs and Southern Kings, joined the then Pro12 in 2017 to form an expanded Pro14 competition, but their involvement has ended.
"This is a huge disappointment, but time had simply run out," said SA Rugby chief executive Jurie Roux.
"No stone was left unturned to try and find a solution to the challenge, including basing our teams for 10 days in locations in the Middle East or Europe.
"The pieces of the jigsaw would not fall into place in time to allow us to put those plans into action."