Beekeepers in northeastern France have been alarmed to find their bees producing honey in unnatural shades of green and blue.

The beekeepers believe the source of the problem is a biogas plant close to Ribeauville in Alsace.

It is thought the bees have been eating the sugary waste from M&Ms, small chocolates in brightly-colored shells.

The plant operator said it regretted the situation and had put in place a procedure to stop it happening again.

“We discovered the problem at the same time [the beekeepers] did. We quickly put in place a procedure to stop it,” Philippe Meinrad, a spokesman from Agrivalor, the company operating the biogas plant, was quoted by Reuters as saying.

The company, which deals with waste from a Mars chocolate factory, said it would clean out the containers, store all incoming waste in airtight containers and process it promptly, according to a company statement published in Le Monde newspaper.

Bee numbers have seen a rapid decline globally in recent years.

As for the blue honey, the beekeepers say it is unsellable.