The charter was issued by Charles II following his exile to Europe after his father, Charles I, was executed in 1649.
ADVERTISEMENTBelgium could turn to the past if Brussels fails to agree a trade deal with London in the present.
European fishermen, including Belgian ones, would lose the right to fish in British waters if there is no EU-UK agreement.
But Belgium could turn to a 17th-century charter in an attempt to keep at least some of its access.
The charter, known as The Privilege, allows up to 50 fishermen from the Flemish city of Bruges access to British waters in perpetuity.
It was issued by Charles II following his exile to Europe after his father, Charles I, was executed in 1649.
Jan d'Hondt, the chief archivist at the Bruges city archive, told Euronews the so-called privilege was given as a gift.
"The Privilege was given by Charles II to the citizens of Bruges," he said. "He gave that in 1666 and why he gave that was because he was here in exile in Bruges from 1656 until 1659 because Cromwell had executed his father... and as a kind of gratitude he gave this privilege to Bruges."
The UK has said, however, other more recent treaties override this old Belgian document, like the United Nations Convention on the Law of the Sea, which grants countries rights to their surrounding waters by up to 12 nautical miles.
But, according to d'Hondt, this may not be true given Britain's attachment to traditions.
"Great Britain is a country with a lot of old traditions and common laws," he said. "They have the Monarch Act of the 13th century and maybe you can say that Privilege is still legal, but it's not up to me to decide that."