The two neighbours both claim 860 square kilometres of the Mediterranean Sea as being within their own exclusive economic zone.
Britain, Europe's most heavily-impacted country by COVID-19, requires anyone coming in from abroad to self-isolate for 14 days.
Before he became prime minister, Boris Johnson had promised businesses in Northern Ireland they would face "no forms, no checks, no barriers."
The coronavirus has so far tested health care systems and economies. But if borders continue to close, it will be a test for the single market, too.
Greek border police fired teargas toward migrants on Saturday as tension continued at a border crossing in the Evros region
French police have used pepper spray and batons to push back pro-Catalan independence protesters who blocked a major highway border pass between France and Spain.
There's speculation that the European Union and the UK government may be close to securing a deal over Brexit. Marathon talks went on into the early hours of Wednesday morning. The discussions have centered on the issue of the Irish border and customs.
Concerns are mounting on the island of Ireland that if a No-Deal Brexit happens, dissident Irish republicans may return to terrorism if customs check points are reintroduced.