Up to 50 Conservative MPs could vote against the Government this week on Sunday Trading, as Labour urges the SNP not to support the proposals.
Rumours have emerged of compromise deals on the issue, but there are concerns these offer no real protections for employees.
The Government wants to give local councils power to extend shopping hours on Sundays, but critics have warned that would pressurise workers and undermine family life. A vote is expected on Wednesday.
Media reports over the weekend suggested up to 50 Tory MPs could stand against their Party, with two ministers reported to be considering resigning over the issue.
David Burrowes, the Conservative MP for Enfield Southgate, is spearheading the opposition and said the Government needed to, “take a long hard look at what they are doing”.
A minister who is considering voting against the plans said the Conservatives’ protections for workers were “weak”.
Another member of the Government said: “There should be a day when people do not have to work. I do not think the economic benefits of this are worth the political cost or the potential human cost.”
Labour is against weakening the laws, and its UK and Scottish leaders have written to the SNP calling for it to reject the proposals.
The extended opening hours apply to England and Wales, but the changes to employment law include Scotland as well. The changes have sparked fears that some UK-wide companies will use the new law to put more pressure on their employees in Scotland to also work on Sundays.
The SNP said before Christmas that it would vote against the idea but now appears to be wavering.
The Christian Institute has commissioned a legal opinion on the subject which highlights serious problems with the Government’s plans.
John Bowers QC, a leading authority on employment law, said the plans offer no protection from detriment or dismissal for people who object to working on Sundays during the opt-out notice period.
He also stressed that the Equality Act cannot be relied on to protect Christians and offers no help to non-religious people who just want to keep Sunday as a family day.
The Christian Institute said the Government’s efforts to allay concerns “offer no real protections for Christians who do not wish to work on a Sunday or people who want to keep Sunday as a family day. The promises are nothing more than a mirage.”