Government plans to loosen Sunday trading laws in England and Wales are facing defeat in the Commons after the SNP indicated it will oppose them.
The party is ready to join forces with Labour and around 20 Tory rebels to block the changes amid fears they could drive down Scottish workers' wages.
Chancellor George Osborne outlined proposals in the Budget earlier this year that would devolve powers to set Sunday trading rules to local councils and mayors.
Currently large stores and supermarkets can only open for six hours on Sundays.
The SNP has apparently been convinced by shopworkers' union Usdaw that retailers would fund extra opening hours by cutting the wages paid on Sundays across the UK.
Angus Robertson, the SNP's Westminster leader, 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."
Mr Robertson said there were on average 3,875 shop workers in every constituency in Scotland, and added that 94% of them who work on a Sunday receive time and a half or double pay.
"If these changes go through as planned in England, there is a very severe concern that that will be negatively impacted and, given the pressure that there is on low-income workers and families, the SNP will vote against these proposals because they do impact on workers in Scotland as they do throughout the rest of the UK," he told BBC Scotland.
He insisted that, while he was a supporter of Sunday trading, it could not be introduced in the rest of the UK "in a way which is detrimental to shop workers" across the UK.
Speaking on the Good Morning Scotland programme, he added: "The Evel (English votes for English laws) issue, I think, has permeated the decision-making in the UK Government so they think they can get through whatever they want that is directly impacting on England whilst forgetting the fact that there are indirect impacts in Scotland.
"Because of that mindset, they have put forward a proposal, and this is the classic example, where they do not understand that it actually has a locus in the rest of the UK.
"Were they to have thought about it, and were they to have made a respectful approach to the issue, to have talked with the SNP, talked about how to safeguard shop workers' rights and pay, we would have been prepared to consider the proposals that were being brought forward. They didn't."