The Government has put plans to let shops open longer on Sunday "on hold" after an "unholy alliance" was formed between the Scottish National Party and Tory rebels.
Sajid Javid, the Business Secretary, was contemplating another Government U-turn after the SNP announced it would vote against plans for more liberal trading hours in England and Wales, bucking convention that says it does not interfere with laws only applied south of the border.
The Government has a slim notional majority of 17 MPs. But the SNP joining forces with Labour, and 20 rebel Tory MPs, threw the proposed legislation into jeopardy - embarrassing the Chancellor who has hailed the reform for boosting jobs.
Tory MP David Burrowes, who opposes extending Sunday trading, told The Huffington Post UK that the Government had "enough battles" as it is, singling out the fall-out over slashing tax credits.
He said: “There are around 20 colleagues who will vote against relaxing Sunday trading laws and with an unholy alliance with the SNP are likely to defeat the Government.
"They share my concern that it was not in our manifesto and is not right for the party of workers, small businesses and families. We have enough battles around issues like tax credits without an unnecessary one over Sunday trading.”
Most Conservatives opposing the plans do so on religious grounds.
The SNP fears wages would be driven down north of the border as retailers seek to offset the cost of rostering extra hours in England and Wales.
The party's Westminster leader Angus Robertson 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 existing law prevents department stores, supermarkets and garden centres opening for more than six hours on a Sunday, but excludes convenience stores.
The Sunday Trading Act, which allowed limited opening hours on a Sunday, never applied to Scotland, where retailers have freedom to open longer hours. Scottish workers are guaranteed a wage premium for working on Sundays.
It is the second time the SNP has opposed this Government's plans not relating to Scotland after effectively blocking plans to relax the foxhunting ban in England and Wales.
The SNP will argue, however, the reforms require amendments to legislation introduced before "English Votes For English Laws" was enacted.