THE HAGUE, Netherlands — The European Union’s police agency is warning nations to be on the lookout for fake COVID-19 test certificates, as crime gangs attempt to cash in on pandemic travel restrictions.

Many countries have introduced requirements for arriving passengers to show a negative COVID-19 test to slow the spread of the coronavirus brought in by people arriving from other nations.

Europol said Monday that recent cases reported by EU member states include a forgery ring selling negative test results to passengers at the Charles de Gaulle airport in Paris and a counterfeiter detained by Spanish police for selling fake test results.

British authorities also caught fraudsters selling COVID-19 documents for 100 pounds ($137) each.