Sainsbury's Local in Borough High Street has reopened after a week's closure for refurbishment and extension.
The supermarket, which has been expanded by eliminating a staff corridor at the side of the shop, now has a trading area of 325 square metres.
Under the Sunday Trading Act 1994, only stores of 280 square metres or less can open for more than six hours on Sundays.
The store, which was previously open 7am-midnight seven days a week, will now open between 12 noon and 6pm on Sundays.
New features of the expanded store include a hot food cabinet and self-checkout tills.
A notice at the shop's entrance warns customers that audio may be monitored and recorded for security purposes.