The pair would be in line to feature for Luis Enrique's side should they reach the final of the Champions League in Cardiff on June 3, but could face difficulties entering the country due to ongoing legal issues in Spain.
Barca will need to stage a comprehensive comeback if they are to remain in the competition after they suffered a 4-0 defeat to Paris Saint-Germain in the first leg of the last 16 last month, but Ceferin is worried that the British government could hold influence over the final should the Spanish side progress.
"Neymar and Lionel Messi both have [tax evasion] procedures going on," he told the New York Times. "This year, the Champions League final is in Cardiff. Imagine if they did not let them in.
"That is a big thing for us, if players from England can travel anywhere, but players from other teams cannot travel to [the UK]. With free movement in Europe, it is much better. If we see that players cannot enter because they have any sort of procedure ongoing then we will simply think if we should play our European matches there."
Earlier this season the UK government denied PSG defender Serge Aurier entry to the country in order to play a Champions League game against Arsenal due to a conviction for assault.