The government is facing defeat over its plans to relax Sunday Trading regulations after the SNP has said it will join Labour and Conservative rebels in opposing the changes.

Although the legislation will only affect England and Wales, the Scottish National Party (SNP) told the BBC and the Guardian it has decided to vote against the changes, claiming they would drive down Scottish workers' wages.

"SNP MPs could hold the balance of power in the House of Commons on Sunday shopping and we will not undermine shop workers," said the SNP's Westminster leader, Angus Robertson.

"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."

Shopworkers union Usdaw claim that retailers across the UK would pay for the extra hours by cutting wages across the UK. The SNP is said to be convinced this would affect Scottish workers.

Around 20 Conservatives, led by Christian MP David Burrowes, are set to rebel and vote against the plans. With a majority of only 12, the SNP's pledge could mean the government will be defeated.

"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," said Burrowes.

"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."

Ministers are discussing whether the proposals might have to be delayed or even dropped after Greg Clark, the communities and local government secretary, met with possible Tory rebels to hear their concerns.

The government wants to devolve the decision making on Sunday trading regulations to local councils and cities. This would allow local areas to decide whether or not to liberalise the restrictions which currently mean large stores can only open for six hours on a Sunday. Smaller high street shops are not limited by the regulations.

"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," Clark told MPs on Monday.

However critics say devolving the decision making will effectively mean that councils are forced to liberalise the law to prevent retail in their local area falling behind nearby competition.

The Church of England has said the plans are "directly contradictory to the Government's desire to build more resilient local communities" and Nola Leach, CEO of the Christian policy charity, CARE, has accused the government of devolving "their responsibility to uphold and protect the family."