Keep Sunday Special's warning comes amid reports up to fifty Conservative MPs could rebel against government plans to extend them in a vote this Wednesday.
On Sunday, it was claimed two unnamed government ministers threatened to quit over the issue.
The effectiveness of a Conservative rebellion depends significantly on the Scottish National Party (SNP). They previously opposed deregulating Sunday trading, but have said they have changed their mind on the issue.
The SNP met with Usdaw, the shopworker's union, on Monday.
The Church, unions and other critics have said extending Sunday trading hours would damage family and community life, and that protections for employees would not work in practice, meaning they could be working longer for less.
The government has said extending Sunday trading would boost the economy and give more choice to shoppers.
It has also said liberalising the laws does not go against Britain's Christian heritage, pointing to other countries like the United States.
Jonathan Ashcroft, from the Keep Sunday Special campaign, told Premier's News Hour: "Scottish shop workers are likely to lose premium payments, and that means that some Scottish shop workers could lose as much as £1,500 a year.
"Now, nothing that the government has done has changed that fact.
"An analysis by Oxford Economics suggested there would be a net loss of jobs in the retail sector of 3000 jobs, which is the consequence of driving trade from some of the small convenience stores to the large supermarkets."
Listen to Premier's Hannah Tooley speaking to Jonathan Ashcroft on the News Hour: