Spanish side Atletico Madrid on Sunday reported two positive coronavirus tests, just four days before they face Leipzig in Portugal for a place in the Champions League semi-finals.
"On Saturday, all members of the first team and the club's party to Lisbon underwent tests as required by UEFA protocols to participate in the quarter-finals of the Champions League," said a club statement.
"Among the results known today, two positives have appeared and they are self-isolating in their respective homes."
It was not revealed whether the two positive cases involved players or backroom staff.
Atletico said UEFA as well as the Spanish and Portuguese football and health authorities have been informed and that a fresh round of tests will be carried out on the squad and support team.
They admitted this could mean last-minute changes to travel schedules as well as accommodation plans in the Portuguese capital.
"The club will coordinate the new schedules with UEFA and as soon as there is a new plan it will be made public," added the statement.
"We also request the utmost respect over the identity of the two positive cases."
The team were due to travel to Portugal on Monday for the closing stages of the Champions League which is being held in Lisbon as a precaution against COVID-19 rather than risk having teams travelling across Europe for home-and-away ties.
The self-contained eight-team tournament will be staged behind closed doors, starting on Wednesday with the final set for Lisbon on August 23.
- Local quarantines -
Spain has so far been one of the countries worst affected by the coronavirus pandemic, with 310,000 registered infections and 28,500 deaths.
Although most sports have returned from a months-long lockdown caused by the virus, there are concerns that numbers of cases are beginning to rise again.
In recent days, Spain has seen local quarantines imposed in the Basque Country, Catalonia and Aragon.
Protective face masks have been made compulsory in several busy areas, especially in the Madrid region.
Last month, Atletico's city rivals Real Madrid announced forward Mariano Diaz had tested positive for COVID-19.
Sevilla, meanwhile, reported an unnamed player had also contracted the illness.
Real have since been knocked out of the Champions League but Sevilla are still involved in the Europa League.
They play English side Wolves in their quarter-final in Duisburg, Germany, on Tuesday.
The Champions League had been put on hold since March because of the virus with the last-16 stage only completed on Friday and Saturday.
In their statement on Sunday, Atletico were keen to reassure football authorities, explaining that they had followed a very strict protocol even after the end of La Liga season on July 19.
Weekly tests were carried out on July 26 and August 2.
For its 'Final Eight' tournament in Portugal, UEFA has planned strict protocols -- each team will have to provide test results before travelling to the country, and will be tested again on the eve of their first match.
Each positive test will require the individual to self-isolate.