People are continuing to slip on the new paving stones in Lancaster city centre.
One resident said three people had slipped near TK MAXX in Market Street during wet weather, and he had slipped previously, bruising his arm.
Mark Pickup, from Lancaster, said: “Three people slipped this morning outside TK MAXX, one quite heavily.
“I have previously slipped here and hurt my arm, and I slipped again this morning but managed to stay up.
“There was a poor young lady who slipped and banged her head.
“I saw an elderly lady fall this morning as well.
“The council seem to have done a bit of power washing, but there’s still a lot of green stuff on the stones and it’s still very, very slippy.”
The new paving was laid as part of the Lancaster Square Routes project in 2015.
Lancaster City Council said that aphids feeding on the trees in the square secreted a liquid onto the surface below, making it slippery when wet.
A spokesman said the council would be investigating further.
He said: “The city and the county council have recently heard some informal reports of people slipping in the vicinity of trees in the city centre particularly following showery weather conditions.
“Slips can be caused for a variety of reasons. However, it is a fact that lime trees in places such as Market Square are a haven for aphids that feed on their sap.
“They then secrete ‘honeydew’, a sugar rich sticky liquid, on to the surfacing below, which can cause it to become slippery in wet weather, and makes it difficult to clean.
“Having considered the future of a number of trees in Market Square following a petition from the public calling for them to remain, the council decided that the trees themselves should stay.
“At the same time the cleansing frequency was increased and the methods and chemicals used changed.
“We will of course investigate further and see if any further practical adjustments in cleansing frequency and method are needed.”