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."

Usdaw general secretary John Hannett said: "On the face of it, MPs in Northern Ireland and Scotland might feel that changes to Sunday trading in England and Wales will have no effect on their constituents, but we are advising that they consider the possible impact on shopworkers' terms and conditions of employment in their constituency.

"Extended Sunday opening will increase retailers' overheads but not the amount they take through the tills, because people don't have more money to spend just because shops are open for longer. Margins in retail are very tight and businesses can ill-afford to increase costs and reduce profitability.

"So we believe that retailers may seek to mitigate the additional costs of opening for longer by reducing shopworkers' pay or changing their terms and conditions of employment to ensure that there is no increase in the wage bill."

Tory MP David Burrowes, who has been campaigning against the changes, said: "There are around 20 colleagues who will vote against Sunday trading laws and, with an unholy alliance with the SNP, are likely to defeat the Government.

"They shared my concern that it was not in our manifesto and it is not right for the party of workers, small business and families.

"We have enough battles around issues like tax credits without an unnecessary one around Sunday trading."

Downing Street denied reports that the plans had been put on hold in the light of the opposition.

The Prime Minister's official spokeswoman said there was "absolutely" no U-turn on the policy - and that next steps would await analysis of the results of a consultation and be made clear "in due course".

Asked repeatedly if a Commons vote had been postponed, she told reporters: "We have been clear what the process is."