The fake molluscs contain a tracking device which is activated by sudden movement, meaning that oyster producers can detect and track thieves.

Emmanuel Parlier, founder of Flex Sense, has seen rentals of their spy kit skyrocket in the festive season so far, The Times reports.

Mr Parlier said he now has more than 400 customers in France, where oyster sales are worth nearly €1 billion a year.

Batches of six or twelve spy oysters are rented for €10 each per month and discreetly deployed among the real things.