Union of Shop, Distributive and Allied Workers (Usdaw), questioned more than 600 of its members working in retail in Northern Ireland. Almost two thirds of them said that they already come under pressure to work on Sundays.
John Hannett, Usdaw general secretary, said in a press release: "Our members in large stores remain absolutely opposed to extended Sunday trading. The number one reason for their opposition is the detrimental effect this would have on their family life.
"Many shop workers, particularly parents, tell us how important Sunday is to them and their family. Often it is the one day of the week when everyone can sit down together for a meal, with many saying they needed the time on Sunday to help their children prepare for the school week."
Belfast City Council is currently consulting on a proposal to designate Belfast as a 'holiday resort' under Northern Ireland's Article 6 of the Shops Order 1997.
Currently, shops that have more than 280 square metres of retail floor space are only allowed to open for a maximum of five hours on Sundays between 1pm and 6pm. They must close on Easter Sunday and Christmas Day, if it falls on a Sunday. Smaller shops can open all day on a Sunday.
If the proposal to extend Sunday trading is introduced, large shops would be able to extend their Sunday trading hours on up to 18 Sundays in any calendar year between March 1 and September 30. They would still have to close on Easter Sunday and Christmas Day, if it falls on a Sunday.
On the Council's site it states: "We're not reviewing any existing laws. We're simply exploring whether or not to designate the council area as a holiday resort under existing legislation.
"We want to find out if this is likely to benefit the local economy and the tourism industry in particular."
The law states that councils may designate any area within its district as a holiday resort, which would allow extended Sunday trading times.
Members of the public can share their views by completing the council's online questionnaire before May 5, 2017.
