Morrisons has been accused of driving business away from Bognor Regis town centre.
The retailer has imposed a pay-and-display regime on the car park of its Bedford Street store.
The new rules mean motorists have to pay £2 for a two-hour stay when they arrive.
The money is refunded when they spend £10 or more in the Bedford Street store.
Failure to comply will see a motorist fined £80 by Morrisons’ parking enforcement contractor.
Jean Coles, the secretary of the BRighter Bognor campaign group, said: “Paying up-front for parking seems to have been applied overnight and without warning.
“BRighter Bognor wonders about Morrison’s methods.
“We can see their wish to charge people who are not their customers.
“It seems heavy-handed and poor public relations when the company has permission to expand and rebuild its town-centre store – and received public approval for those plans.”
Customer Graham Hutchings said: “I find it amazing that Morrisons, who want to expand their business in Bognor, would implement a system that is obviously flawed and potentially penalises their genuine customers by charging them excessive fees to park.
“If I get a fine through my door, not only will I contest it, I will happily boycott Morrisons and take my business elsewhere, amounting to thousands of pounds of trade each year.”
Customers had been able to park in Morrisons for free for two hours if they spent at least £5 in the store.
They showed their receipt at the exit barrier, but people could park for free if it was unmanned.
The company turned the 175-space car park into a pay-and-display site at the start of last week.
Its parking operator, ParkingEye, is responsible for enforcing the rules.
The change was announced through a series of notices around the car park and in the store’s windows.
They state: “This car park is private property and for the use of customers only while shopping in store.
“Parking limited to two hours. No return within one hour.”
They warn the rules are in operation around the clock.
A Morrisons spokesman said: “We found some motorists were abusing the previous parking regime.
“We had quite a few complaints from customers about that and brought in the new system as a last resort.
“It means there is enough room for our customers.”