In a letter addressed to "guys" the company said "operatives who work in a dusty environment - all of ours - must come to work clean shaven and able to wear appropriate dust masks effectively".
It said the health and safety regulator took "a strong stance on this, as do Mears".
Exceptions might be made for workers who could not shave for medical reasons, a dust mask could physically not be worn or if a beard was displaced for religious purposes.
"Even in [these] circumstances, this is not guaranteed," said the note.
Goatees were a possibility but only if said masks could be correctly donned over them.
Any employees who flouted the facial hair decree could be "taken down the disciplinary route."
However Unite, a major British trade union, said Mears' beard ban was "hair raising in its arrogance".
Regional official for London, Mark Soave, said facial hair was "a highly delicate issue" where sensitivity should be key.
"This is clearly a case of Mears going for the cheapest option and amounts to penny-pinching stupidity.
"Other forms of masks are available and these should be offered to existing workers," he said.
"Unite will always put the safety of our members first and creating huge resentment and anger among your workforce is never the way forward."
But the company is having none of it, The company's group health and safety director,
Mark Elkington said the company had a legal responsibility to ensure that employees' safety, reported The Guardian.
"The simple fact is that no dust mask can work effectively unless it forms a seal against the skin.
"That is not possible with a beard or even heavy stubble. If the Health and Safety Executive (HSE) did a spot site visit and found workers wearing dust masks that were not sealed against the face then we would be liable to prosecution."
The HSE do indeed recommend that dust masks may not fit properly or work as well if a bushy beard gets in the way.