The group leader of the SNP Angus Robertson said his party's decision to vote down changes which will crush the Tory's Sunday trading plan is purely to protect Scottish premium pay.
Scots currently receive a better hourly rate for working on a Sunday, however UK-wide chains who may want to open for longer in England and Wales might want to off-set the cost of their new trading hours by scrapping the premium pay deal.
This could be the second time the nationalist party effectively kill off a Government proposal, following on their decision to vote against relaxing the hunting ban last summer.
And until the Government promises they will protects Scottish workers premium pay, the SNPs 54 MPs will back Tory rebels to vote down the reforms.
Mr Robertson said: "Because of the Government's precarious position, and today is an example that reminds us they only formally have a majority of 12 if there's ever a rebellion or split in parties, it means that the SNP votes at Westminster have a direct impact."
"There's an impact if we vote and an impact if we don't vote. So people have to listen to what we have to say, both in Government and Parliament."
The MP for Moray, who leads the SNP in Westminster, said: "There is time to bring forward a proposal. Sometimes staring down the barrel of a defeat in the House of Commons can be quite helpful in deciding there are things they could be doing they previously thought weren't possible. I would encourage them to do that."
Since 1994, small shops up to 280 sq m in England and Wales can open when they want to on Sundays but larger stores like supermarkets can only open for six hours between 10am and 6pm.
However the new rules allow for longer hours and give local councils control over the opening times in England and Wales.
Mr Roberston denied blocking the plan was a political stunt and contradictory to the SNP's stance on local communities being at the heart of decision making.
He said: "Everything in politics is political. I'm a politician and the SNP is a political party and as a political party we are elected to represent the interests of our constituents. Our constituents will be impacted were this bill were to go through without safeguards on premium pay.
"Is that political? Yes it is. It's called representing your constituents in a democracy.
"Am I ashamed of that? No I am not, it's my job. I am elected to come here and represent the interests of my constituents and that's exactly what I am doing."
"Are people really suggesting we should sit on our hands, and not vote on something that will have an impact on our constituents?
"Is someone seriously suggesting that? Whether in the fourth estate or anywhere else in Westminster? At that point it usually goes quiet...
"This is a measure that will impact workers in Scotland, we know that because the reports have been produced by the Scottish Trade Union Congress, Usdaw (Union of Shop, Distributive and Allied Workers), by academics. There is no dispute about the fact that there is a risk to premium pay.
"That is the reason why we are opposing it. Not because we have an issue with Sunday trading, or an issue with local decision."
He said that the only 'politicking' at play was the Government's decision to ignore pleas to protect Scottish premium pay.
He said: "What disappoints me though when talking about politics in the sense of 'politicking' is that when you have serious conversations and you explain to Government there are ways in which Sunday trading can be introduced in a way that's not detrimental to workers in Scotland, that people chose to ignore that."