Four estate agencies have been taken to court over allegedly illegal boards.
Among the four that Colchester Council prosecuted was Spicerhaart – which is headquartered in the town.
The council also brought charges against William H Brown, Saxons and David Martin Group.
The council said the signs had been put up as advertisements and did not relate to properties for sale.
Each estate agent except Saxons admitted the charges.
At Colchester Magistrates Court, Claudia Lord-Lynch, prosecuting, said the council had sent letters in 2015 to each business, warning of legal action if regulations were breached.
A further letter was sent last November.
But the court heard that in February unauthorised To Let and Sold signs were discovered on communal grounds.
Jeff Hamblion, senior property manager at Saxons, denied the charge, saying: “We have photographic evidence of where it [the sign] should be and where it has been positioned in the past.
“We believe it is some drunk, or someone else, who has come past and removed it from its position of where it should be and left it in the position it was found in February.”
The company was summoned to appear for a trial on August 17.
Nick Price, for David Martin Group, said: “In 22 years of operating, we have never been asked to remove a board. It was simply one that slipped through the cracks and on this occasion it was not picked up.”
David Martin Group, William H Brown and Spicerhaart were all ordered to pay a £500 fine, a victim surcharge of £50 and prosecution costs.
A Colchester Council spokesman said: “We hope that bringing these cases to court will serve as both a deterrent and a reminder to estate agents to think twice before leaving unnecessary signs up at properties or on public land for which they have no permission.”