French unions on Thursday unfurled a giant banner opposite English shores to goad Britain's King Charles III after his first foreign state visit to France was cancelled.

"Sorry Charles, see you later," said the banner unfurled by union activists a week after President Emmanuel Macron postponed at the last minute the planned visit by Charles.

Macron said that nationwide union-led protests and strikes against contentious pension reforms prevented France from hosting Charles in the manner it had hoped.

With the trip postponed, it fell to Germany to host Charles for his first foreign trip as monarch, where he was received with pomp on Wednesday and Thursday.

Around 100 French union members unfurled the banner on Cap Blanc Nez, a point outside the northern city of Calais that is one of the closest places in France to England.

They battled clifftop winds but succeeded in unfurling the banner so it could be easily seen from the coast.

The postponement of the visit meant that Charles had to forgo plans for a state banquet at the Palace of Versailles outside Paris and a trip to the southwestern city of Bordeaux.

Macron has said the visit could go ahead in the early summer, but it remains unclear if Charles will find space in his schedule.