The plan, to be debated today, was to be under English Votes for English Laws (EVEL) rules disallowing SNP votes but that is now not the case.
There are around 30-50 Conservatives that could vote against the extension to Sunday opening hours from six hours in England and Wales, a move that could generate £70m a year for garden centres. Scotland already has extended opening.
Failure to win the support of the SNP would all but guarantee the Enterprise Bill proposal would be defeated in the House of Commons on Wednesday given there around 50 Tory backbench rebels.
The SNP’s 56 MPs meet in Parliament on Tuesday night to decide a formal position on the proposals.
If certain safeguards are put in place SNP MPs are set to abstain in the vote, the BBC has suggested, adding that SNP may will not block the measures provided higher Sunday pay rates in Scotland are protected and other safeguards for employees are introduced. The SNP deny this.
An SNP representative said amendments to the bill may have meant that EVEL rules did not apply to some aspects of it. The SNP added that SNP MPs will decide which way to vote tonight (Tuesday) but their position "has always been to protect the rights of Scottish workers".
SNP Economy spokesperson and Deputy Leader Stewart Hosie MP said: "Protecting Scottish workers will be paramount to our decision on Sunday Trading. The SNP has continued to engage with lobby groups and trade unions. MPs have been looking at all sides of the debate and a decision will be made when we meet this evening, ahead of the vote on Wednesday."
In November HTA Scottish representative Neil Cummings wrote to newspapers saying: "It is surprising and disappointing that the SNP, which is happy to allow shops and garden centres here in Scotland to open for a full day on a Sunday, has announced it will seek to block the UK Government’s plans to give the powers to local authorities in England and Wales to enable the same opening hours there.
"Whatever the merits of the political tactics employed on behalf of the SNP in Westminster, the claim that Scottish workers will have their pay cut as a result is without foundation. Premium wages for working on a Sunday is an exception and not the rule in Scotland, and Scottish workers will continue to receive the same pay regardless of any locally introduced changes to Sunday working patterns in England and Wales."
He added today: "Clearly very disappointed and will continue to engage with SNP and highlight the benefits that our Scottish retail members and the community are experiencing by being open longer on a Sunday."