The UK's Maritime & Coastguard Agency (MCA) is having to reassess about 500 seafarers after discrepancies were discovered with a course provider.
The crew in question completed safety-critical courses governed by the requirements of the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW).
Affected seafarers will be contacted within five working days.
Katy Ware, MCA director of Maritime Safety & Standards, said: "As a flag state, the MCA takes its role very seriously. We appreciate this may cause some people anxiety which is why we’re contacting them as soon as possible.
"We are required by law to make sure training establishments, which provide courses that underpin issuing of UK Certificates of Competency to individual seafarers, comply with the training, assessment and quality standards set out by the International Maritime Organization (IMO)."