The UAE, along with Saudi Arabia, Bahrain and Egypt, cut all ties with Qatar in June 2017 over allegations that the tiny, energy-rich state supports Islamic extremists.
The Foreign Office warns in its advice for visitors to the UAE of subsequent new laws.
“Showing sympathy for Qatar on social media or by any other means of communication is an offence,” it says.
“Offenders could be imprisoned and subject to a substantial fine.”
The UAE hosted the recent Asian Cup 2019 football tournament. Qatar beat Japan 3-1 in the final on Friday to win the competition for the first time.
Qatar thrashed the UAE 4-0 in the semi-finals when their players were pelted with shoes and plastic bottles.
The Guardian reported that Ali Issa Ahmad, 26, had travelled to the UAE for a holiday, bought a ticket for the second round match between Qatar and Iraq on January 22 and wore a Qatar shirt to the game.
“We are providing assistance to a British man arrested in the UAE, and are in touch with the local authorities,” a Foreign Office spokesman said.