All passengers who were on a flight to Wales from the Greek island of Zante have been told to self-isolate after some of those travelling tested positive for Covid-19.
At least seven positive cases have been identified among three different parties on the Tui flight 6215 to Cardiff on August 25, according to Public Health Wales.
Health officials said all passengers on board the flight are now being considered close contacts and must self-isolate.
Dr Giri Shankar, of Public Health Wales, said passengers were being contacted but must quarantine at home in the meantime, as they could become infectious even without symptoms.
"Cardiff and Vale Test Trace Protect and Public Health Wales have identified at least seven confirmed cases of Covid-19 from three different parties who were infectious on Tui Flight 6215 from Zante to Cardiff on 25 August," he said.
"As a result, we are advising that all passengers on this flight are considered close contacts and must self-isolate."
No new deaths related to Covid-19 were reported by the public health body on Sunday, while it said 56 cases had been confirmed.
Dr Shankar said that an investigation into positive cases in Wales had indicted a lack of social distancing between those aged between 20 and 30 had "resulted in the spread of the virus to other groups of people".
He urged revellers enjoying the bank holiday weekend to remember the importance of social distancing.
"I would make a direct appeal to young people to remember that even if they feel that they would not be badly affected by Covid-19 if they were to test positive for it, if they were to pass it on to older or more vulnerable family members, friends or colleagues it could be extremely serious, even fatal," he added.