Government plans to relax Sunday trading laws in England and Wales are facing defeat in the House of Commons.
The SNP has told the BBC and the Guardian the party has decided to vote against the changes amid fears it could drive down Scottish workers' wages.
With other opposition MPs expected to join forces with some 20 Tory rebels, the plans will struggle to pass.
Ministers are said to be considering whether the proposals might have to be delayed or even dropped.
The chancellor, George Osborne, promised in the Budget earlier this year that councils and mayors would get the power to set Sunday trading laws in their areas.
Large stores and supermarkets can currently open for only six hours every Sunday.
'No pay safeguards'
Although the legislation affects only England and Wales, the SNP has been convinced by shopworkers' union Usdaw that retailers would pay for the extra hours by cutting wages across the UK - including those of Scottish workers currently being paid premium wages for Sunday work.
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."
The government launched a consultation over the summer to look at the changes which are designed to support High Streets and shopping centres by allowing them to stay open for longer at weekends and compete with internet shopping.
Shops in Scotland already have more freedom because the Sunday Trading Act does not apply north of the border.
Government sources pointed out that Scotland already controls its Sunday trading rules and the SNP was once again trying to block something that had no impact on their constituents. They said the SNP move once again made the case for the government's recent changes to parliamentary rules that gave English and Welsh MPs greater control over laws that affect only their constituencies.
It is not yet clear which measure the government will use to introduce the changes - either the Cities and Local Devolution Government Bill or the Enterprise Bill.
Conservative MP David Burrowes, who opposes changes to Sunday trading laws, 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 on Monday: "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."
He added: "There is a consultation out on this at the moment, but the proposal in the consultation is to allow local councils to make those decisions.
"Councils have a wide remit, social as well as economic, to look after the interests of their area.
"It could be that allowing some areas or particular stores such as garden centres to open on their busiest day - Sunday - is in the interests of everyone in the area."
'Deregulation by the back door'
Shopworkers' trade union leader John Hannett urged the government not to proceed with the changes.
He said respondents to the government's consultation expressed "a great deal of opposition" to the proposals, "because devolving these regulations will not provide help to local businesses, but will create chaos for retailers and shoppers as every local authority will have different Sunday trading hours".
He added: "Our members working in retail are extremely worried that devolution will lead to deregulation by the back door, and that is why 91% of workers in large stores oppose the proposal.
"We are aware that there is a great deal of concern and opposition among MPs from all parties.
"It would be wrong for the government to bring forward an amendment to the Devolution Bill, when Parliamentarians have not had the opportunity to read and digest the views of consultees and the government has not justified their position by issuing a reply, which is standard procedure."