Cardiff Rugby are unable to leave South Africa following two positive COVID cases - one of which is thought to be the new Omicron variant.
The team are now isolating at their hotel.
They had been due to take part in the United Rugby Championship on South African soil this weekend, but all four of the tournament's games were postponed.
The Cardiff Rugby players and those travelling with them had hoped to leave Cape Town on Sunday afternoon after securing a charter plane, but two of the PCR tests they took came back as positive.
"As a consequence of one of these results suspected of being Omicron, the entire travelling party have returned to their hotel to isolate," the team said in a statement.
Munster Rugby, one of the rival teams in the tournament, have also been held back in South Africa after the squad's PCR tests showed one positive case.
"In taking all precautions and prioritising the health and wellbeing of everyone Munster Rugby will not travel today and will return to their hotel for an isolation period as a matter of priority," the team said in a statement, adding that they are awaiting further guidance from health officials.
The team had been making plans to take a charter plane home.
South Africa was among several African countries placed on England's red travel list after the emergence of the Omicron variant.
Direct flights from the country to the UK were suspended from noon on Friday until 4am on Sunday, when a requirement for new arrivals to quarantine in hotels came into effect.
Cardiff Rugby said they continue to work with the South African Rugby Union, the Welsh Rugby Union and public health authorities both at home and abroad to decide the next course of action.
"The club are also continuing to work with all relevant authorities to secure the travelling party's return to Wales when safe and appropriate," the team said.
"Everybody concerned in this highly challenging situation would like to thank their families, friends, colleagues and the wider rugby family for their many messages of concern and well wishes."
The four round-six fixtures - Scarlets v Sharks, Cardiff v Lions, Munster v Bulls and Stormers v Zebre Parma - were called off due to concerns about the Omicron variant, which could have the potential to evade immunity built up by vaccination or prior infection.
Round seven fixtures, due to take place in South Africa next weekend, have also been put on hold.
Zebre's chairman Michele Dalai tweeted a picture of his team at the airport, confirming that they were set to leave the country.