Prince Joachim, 28, nephew of King Philippe of Belgium, attended a social event with the family of a Spanish friend, his lawyer's office at CNN confirmed on Monday.
"I would like to apologize for having traveled and not respecting the quarantine measures," said the prince in a statement issued by the Cordoba law firm of Mariano Aguayo.
"I had no intention of offending or disrespecting anyone in these very difficult times, I deeply regret my actions and accept the consequences."
The prince had traveled from Belgium to Spain on May 24 and attended a social gathering with "12 or 27 guests" on May 26, the palace press office said. He tested positive for Covid-19 two days later.
Under Spain's state of emergency, travelers arriving in the country, with a few exceptions, are required to quarantine for two weeks.
In addition, gatherings of more than 15 people who do not live together are currently not allowed under the containment rules in force in Cordoba.
Authorities are currently investigating the presence of more than 15 people at the social event the prince attended, spokesperson for Rafaela Valenzuela, the main government official in Cordoba, told CNN on Monday.
The spokeswoman added that regional health authorities had informed the Spanish government that there could be up to 27 people at the event.
The Spanish police have the power to convict individuals who break the rules of detention. The amounts vary across the country, but several hundred thousand fines have been imposed since the start of the state of emergency on March 14.
But a spokeswoman for the prince's office told CNN that there had been two social events on consecutive days last week in which the prince had met the family of a friend. Neither event brought more than 15 people together, she added.
The prince is now isolated with mild symptoms of coronavirus, the spokeswoman said.
Prince Joachim is the third child of Princess Astrid, King Philippe's younger sister.
Spain was one of the European countries hardest hit by Covid-19, with more than 239,000 confirmed cases and more than 27,000 deaths, according to data from Johns Hopkins University.
As the infection rate has declined, the government has slowly relaxed containment restrictions, while focusing on early detection through increased testing and contract tracing to quickly eliminate any new outbreaks.
CNN's Abel Alvarado, Laura Perez Maestro and Helena de Moura contributed to this report.
