Scarlets players and staff will continue to quarantine in a Belfast hotel for 10 days.
The 47-person travelling group have been there since Monday after flying back from red-listed South Africa.
Scarlets had hoped to be able to complete their quarantine in Wales but were informed by the Welsh government they must do so in their Belfast hotel
Cardiff remain in Cape Town after positive Covid-19 cases, while Scarlets' tests were all clear.
A Scarlets statement on Tuesday read: "Scarlets have today received a letter from Dawn Bowden, the Welsh government's Deputy Minister for Arts and Sport, confirming that they must see out the remainder of their Covid-19 quarantine period in Belfast.
"The travelling party of 47 have been at the government-managed facility since early Monday morning following their return from South Africa and will complete the designated 10-day isolation period in Northern Ireland, which ends on Friday, December 10.
"Scarlets will continue to adhere to the testing regime set out for this quarantine period.
"Since the decision was made for the group to return from South Africa, we have been in continual discussions with the Welsh government about completing our quarantine in Wales, but have been told that current legislation makes this option impossible because there is no quarantine facility in Wales.
"While disappointed, our utmost priority is the health and well-being of our players and staff. Club officials are in constant contact with the coaching team, players and support staff over in Belfast and we will continue to ensure they have everything they need while respecting the rules of the quarantine process."
On Monday, Scarlets director Ron Jones had called on the Welsh government to help the region's isolating players and staff return to Wales.
But Welsh health minister Eluned Morgan said there were no hotels in Wales which could accommodate the Scarlets' travelling party, adding that new legislation would need to be passed to allow that to happen anyway.
Current rules in Wales state elite sports people are not exempt from isolation upon return from red-list countries and they are not permitted to leave their place of isolation for training.
Like Cardiff, Munster and Italian side Zebre, Scarlets had been in South Africa to play United Rugby Championship matches when the new Omicron Covid variant emerged, prompting UK and Welsh governments to implement new travel restrictions.
They include the requirement for people arriving after 04:00 GMT last Sunday morning from red-list countries, including South Africa, to spend 10 days in hotel quarantine. Scarlets arrived in Dublin on a charter flight at 02:15 GMT on Monday.
Scarlets are scheduled to visit Bristol on Saturday, 11 December in the Heineken Champions Cup.
Speaking on BBC Radio Wales on Monday, Scarlets chairman Simon Muderack accepted that fixture may now be in doubt.
"Time is ticking, the reality is we haven't played a game as a team for quite some time," he said.
"Our last game would have been at the end of October and here we are now essentially into December and every day that the boys are in quarantine, they are deconditioning.
"So we are starting to get to a point, if we are not already at a point, where there is, to be blunt, some personal health and safety issues where you've got a bunch of boys who haven't played rugby for six weeks, who may well be confined to a hotel for 10 days, going up a against a set of finely-tuned athletes who've not skipped a beat in terms of game-time, preparation, nutrition, access to sunlight and being able to spend time decompressing at home with their families.
"There's a set of challenges there that we can't ignore but that's to pick up through the rest of the week."