Unite has criticised the agency in leaflets, press materials and a national newspaper comment piece. The criticism focuses partly on the agency's work for Grangemouth owner Ineos, with which Unite has been locked in an industrial dispute.
The press material included a warning that Media Zoo "could face reputational damage by working with such a ruthless company [as Ineos]".
"Unite should engage with the message, not shoot the messenger," asserted Media Zoo managing director Rachel Pendered.
Unite described the agency as "child labour PR specialists", which Pendered rejected as "distorted and offensive". She also claimed Unite's warning about "reputational damage" was an attempt to intimidate the agency.
"The bully-boy tactics and personal attacks are unsavoury, unnecessary and undignified," she added.
Pendered claimed she was the subject of personal attacks from Labour MP Tom Watson.
Last week Watson blogged "I'm sure Ms Pendered is pulling good money for moving in such esteemed circles. Her former colleagues from BBC show Watchdog will be proud of her success [at Media Zoo], I am sure".
In the same blog Watson wrote the agency "seems as oleaginous as the oil in the refinery pipes" and reproduced a Unite press release titled "Child labour PR specialists help Ineos swindle Scottish workers".
Unite’s child labour claim appears to originate from a line on the agency's website that reads: "Our team has extensive experience of dealing with crisis situations including industrial disputes, fatal accidents, profits warnings, child labour, product failures, customer service issues, redundancies and restructuring."
Media Zoo creative director Mark Killick said the agency had helped a retail client with crisis comms when the client, who he declined to name, discovered one of its suppliers had been using child labour.
Unite confirmed it had based its claim on the words on Media Zoo's website.
Unite chief of staff Andrew Murray responded: "Ms Pendered should understand that PR is not a values-free profession. That is not the way the world is today. Like anyone else, PR firms have to make moral choices as to who they do business with and how."