Usdaw General Secretary, John Hannett, said he is concerned that Sunday trading reform is an answer to disappointing high street trading results over the Christmas period.
He said that: "Indications so far suggest that some retailers have struggled over the Christmas period with sales in shops, most have cited unusually warm weather as the main problem. So, it is very concerning to hear suggestions that longer opening is the answer.
"The last thing retail needs at the moment is a race to the bottom, with 24-hour opening 7 days a week. For large retailers, Sunday is the most profitable day of the week, simply because their hours are restricted.
"Increase Sunday trading hours and overheads will rise significantly, with no guarantee of more revenue through the tills.
"The race to open longer has been a folly and we are now seeing some larger retailers pulling back from 24-hour trading Monday to Saturday. Small retailers are currently free to open any hours they like and changing the law for large retailers will see small businesses lose that slight competitive advantage.
"If the Government and local councils are serious about helping high street retailers, particularly smaller enterprises, they should be addressing crucial issues like expensive rents for retail space, burdensome business rates and excessive parking charges that drive customers away from town centres to out-of-town retail outlets where parking tends to be free of charge.
"The Sunday Trading Act is a great British compromise, which has worked well for over 20 years and gives everyone a little bit of what they want. Retailers can trade, customers can shop, staff can work; whilst Sunday remains a special day, different to other days, and shop workers can spend some time with their family."
At present, stores that are over 280 square metres are allowed to open for six continuous hours in England and Wales.
Small stores do not have any restrictions on Sunday opening hours.
