The SNP has said it is to vote against UK government plans to change Sunday trading laws in England and Wales.
Proposals to allow English and Welsh councils to permit larger stores to open for more than six hours will not directly affect Scotland, where there are no trading restrictions in place.
But the SNP said it had concerns over the knock-on effects on Scottish pay.
With more than 20 Tory MPs set to rebel, the risk of a government defeat in a vote on Wednesday has increased.
MPs will vote on proposed changes to Sunday trading laws in what would be the biggest shake-up for 20 years.
SNP MPs met in Westminster met on Tuesday to decide whether to order their 54 MPs to abstain in the vote, effectively allowing the proposals to pass, or to oppose them outright. They have decided to do the latter.
With Labour also opposing the changes and at least 21 Conservative MPs threatening to rebel, the SNP's stance could tip the balance in the vote.
At the moment in England and Wales, small shops - up to 280 sq m, or 3,000 sq ft in size - can open when they want to on Sundays but larger stores are restricted to six hours between 10:00 and 18:00. Retailers can be fined up to £50,000 if they break the rules.
The UK government's plans to give councils the freedom to determine opening hours will not apply directly to Scotland but the SNP has concerns about their impact on existing Sunday wage rates and people who do not want to work.
Stewart Hosie, the SNP's deputy leader, said the UK government had made concessions to protect workers but they did not go far enough in terms of safeguarding the "premium pay" rates he said Scottish workers currently enjoyed, control of which the SNP wants to be devolved.
"Protecting Scottish workers has been paramount to our decision to oppose the government's plans on Sunday Trading," he said. "We have had Sunday Trading in Scotland for some time and the SNP has never been opposed to it.
"However, our concerns here are rooted in the knock-on impact to Scottish workers who would be at risk of pay cuts - many of whom are already suffering from George Osborne's cuts to tax credits and other in-work support.
"SNP MPs have considered all sides in this debate and our decision has been made so workers in Scotland are protected."
The decision was welcomed by unions, which said "common sense had prevailed".
"Shopworkers across the UK will be relieved the SNP have eventually made the right decision to oppose Sunday trading changes that could impact on shop workers pay," said John Hannett, the general secretary of Usdaw.
Asked before the SNP announcement whether it was confident of Commons approval, Downing Street said it was clear about the economic benefits that could result from giving communities the power to determine opening hours.
"We think this is a way to enhance the ability of communities to support their High Streets to deal with some of the pressures that they face from the online market that we have these days and where we haven't updated Sunday trading rules to reflect that," a No 10 spokeswoman said.
Ministers say the change will help High Streets struggling to compete against online retailers and reflects changing leisure and working patterns.
But critics have said Sunday should remain a special day for people to spend with their family or pursuing other activities, including religious observance, free from added commercial pressure. They have expressed concerns that despite promised safeguards, staff will come under increased pressure to work from employers, and pointed out the proposals were not in the Conservatives' election manifesto.
In the run-up to the vote, 21 Conservative MPs signed an amendment to the Enterprise Bill opposing the changes.
Labour leader Jeremy Corbyn has warned that the proposals would be the thin end of the wedge and could herald a full "seven-day society".
"If we march through those lobbies together tomorrow with a group of Conservatives who share our views - we will win and we'll keep Sundays as a different day - it's worth fighting for," he said during a visit to east London.
"For those with a strong Christian faith who wish to have a different Sunday I absolutely support them. For those who just want a different day of a week on a Sunday, a day they can get together with family... relaxation time, that's good."