Police are searching for the person who damaged the historic Quakers Walk gates.
The lower part of the gate on the left was damaged, including the recently restored roundel containing a dolphin emblem.
The damage is believed to have been caused between 4pm last Thursday and 8am last Friday.
Whoever caused the damage left the broken pieces of the gate by the gates.
PC Cathy Byers said: “The gates are, in fact, structurally safe but they have clearly been damaged. There were shards of yellow plastic there, with blue paint on, which could have come from a trailer – I don’t think they are from a vehicle bumper.”
The gates, which date back to the 1800s and are Grade II listed, were restored last year in a project for Wiltshire Council and paid for by Taylor Wimpey and Persimmon, which are building the new houses nearby.
The restoration work included reinstating the dolphin emblems in the roundels which had been absent for more than 40 years.
The dolphins were the symbol of the Colston family crest. The Colstons owned Roundway Park and the Quakers Walk gatehouse.
Sue Evans, Wiltshire councillor for the area, said: “I’m horrified at the damage and I can’t believe that whoever did it has not owned up. If it was an accident you just can’t walk away, you could leave your name and address. The gates are part of our heritage.”
Philippa Morgan, co-ordinator of Quakers Walk Protection Group, said: “It’s very distressing to all of us who have seen the gates refurbished and put back in the last few months to see them damaged. It’s not obvious looking at the damage what went wrong. It’s also very disappointing the person who did it has driven away.
“We keep our fingers crossed that Wiltshire Council will have it put back as it was.”
A Wiltshire Council spokesperson said: “We have collected the damaged pieces from the gate and it will be repaired very soon.”