Government plans to devolve Sunday trading laws to local councils and mayors look set to be defeated in the House of Commons.
The SNP has made clear its intentions to vote against the proposals. And with 20 Conservative MPs believed to be intent on rebelling, the government could face a tough task to get the law to pass.
Earlier this year, the Chancellor George Osborne said local councils and mayors in England and Wales would be given the power to set the opening hours on Sundays in their area.
Currently, larger stores and supermarkets can only be open for six hours. And even though the law change would not affect Scotland, the SNP is fearful that it would affect pay for Scottish workers.
Shopworkers' union, Usdaw, has told the SNP that retailers would be forced to cut wages across the UK in order to pay for longer opening hours.
Angus Roberston, who leads the SNP in Parliament, said: "SNP MPs could hold the balance of power in the House of Commons on Sunday shopping and we will not undermine shop workers.
"This legislation will impact on workers in Scotland and elsewhere in the UK and no pay safeguards have been offered by the Westminster government.
"The SNP will continue to work with the representatives of shop workers and we will oppose the Tory proposals."
David Burrowes, a Conservative MP planning to vote against the changes, said: "The government is set to embark on a de-regulation of Sunday trading for which there is no particular demand, which was not in our manifesto and goes against our concerns for workers for small businesses and families.
"Some 20 of my colleagues are opposed to these changes and that is more than enough to overcome the government's majority now the SNP have joined the opposition."
Communities and Local Government Secretary Greg Clark told MPs: "The government believe that there is a strong case for local areas to be able to decide if and where extending Sunday trading should be permitted.
"It could help some High Streets compete with online shopping, for which Sunday is regularly the most popular day."