A showdown over Sunday trading is set to occur in the House of Commons in early March after a committee of MPs passed the government's last-minute amendment to the Enterprise Bill on Thursday.

Labour has said it will oppose the change which would allow local authorities to relax restrictions on opening hours for large stores on a Sunday. With over 20 Conservative MPs set to rebel against their own party, the changes could be defeated if the Scottish National Party (SNP) join the opposition.

However there are hints the SNP may have done a deal with the government. When the proposals were first suggested in November, Angus Robertson, the SNP's Westminster leader, said the "legislation will impact on workers in Scotland and elsewhere in the UK". He promised: "The SNP will continue to work with the representatives of shop workers and we will oppose the Tory proposals".

But Hannah Bardell, SNP spokeswoman on business, has now said her party would reconsider the concerns before deciding whether to oppose. This has raised the suggestion among campaigners the SNP might withdraw their opposition, despite research showing Scottish shopworkers would lose up to £1,400 per year under the change.

The SNP were contacted for a comment but declined to do so.

The government has applied the principle of English votes to English laws (Evel) to the amendment, which was introduced late to the Enterprise Bill, meaning neither the House of Lords nor the House of Commons have debated the measure fully.

Evel allows a grand committee of English and Welsh MPs to veto the votes of Scottish MPs on legislation only affecting England and Wales.

However the principle will have no effect on the Sunday trading amendment because it was introduced so late to the proceedings.

As a result, the whole House of Commons will vote on the changes with the SNP holding the balance of power to decide whether or not to support the Conservatives' policy. A vote is "almost certain" after a counter-amendment was tabled against Sunday trading.

In addition to opposition within parliament, a legal challenge has been launched against the government by a number of organisations within the Keep Sunday Special (KSS) campaign.

They have written a 'letter before action' threatening judicial review, questioning the government's consultation which they called a "sham".

"Evidence that does not the government's agenda has been ignored" and the evidence "quoted is irrelevant and outdated", a statement said.

"The consultation amounts to an advocacy document for the proposals instead of being a balanced account of the views expressed by respondents to the consultation, suggesting that the government had made it decision on this policy before considering consultation responses," the statement continued.

A spokesman for the Keep Sunday Special campaign said: "We do not enter into this action lightly, and do so with a heavy heart.

"There are fundamental flaws in the process that the Government has taken and full consideration is needed, not the inadequate process that has taken place to date."

KSS are not the only ones questioning the government's approach. Christian public policy charity CARE, who have campaigned against the changes, called it a "lamentable lack of proper transparency".

"The list of procedural failures are longer than your average Sunday shopping list," said CARE's CEO Nola Leach.

"Not only has the Lords been denied the chance to properly scrutinise the plans, but the Commons have been denied a Second Reading debate on extending Sunday trading.

"Instead the government deliberately shoehorned the proposals into the Enterprise Bill in the full knowledge that doing so would deny Peers and MPs adequate opportunities to examine the proposals," she added.