The bodies have joined together to condemn the recent incident of a particular letting agent, Sutton Estates of Liverpool, which used signs placed outside properties to indicate which of his tenants were not paying their rent.
Head of operations for Arla Ian Potter described the activity as "almost certainly" illegal, while chairman of the NLA David Salusbury called on buy-to-let investors to seek legal advice.
"The law is on the side of landlords in this situation but the courts can be slow to deliver, with delays of six months not uncommon," Mr Salusbury said.
Neil Heffey, a partner at Sutton Estates, claimed that his company had employed the tactic of using signs to humiliate tenants because all other avenues had been exhausted.
