Spencer Grady, who runs a jewellery store on Market Avenue said the higher charges had been “putting people off”.
“They’re coming in here for a repair but they’re not staying for a coffee,” he said.
“They’re just going to the bank or the post office, then running back to their cars.
“The damage to footfall has already been done.”
The new car park tariffs are set to come into force on 12 September and will be reviewed after six months.
A council report said the aim was to support local traders and the community for a limited period of time whilst the redevelopment of Market Square is ongoing.
The project, funded by £20m of Levelling Up cash awarded by the previous government, will be spent on a huge canopy for outdoor traders.
The cash will also be used to help preserve Ashton Town Hall and spent on the old bus station, which closed in 2020.