Christian Tory MPs have said they will form an "unholy alliance" with the SNP and Labour to defeat the government's plans to liberalise Sunday trading rules.

The House of Commons will vote on the proposed changes this evening with at least 23 Conservative MPs set to rebel.

The SNP initially opposed the measures when they were first introduced in November, resulting in an embarrassing climbdown from the government. However they were re-introduced as a last minute amendment to the Enterprise Bill in February, prompting rumours of a "dirty deal" between the SNP and Conservative leadership.

Last night the SNP confirmed they would oppose the changes which would be the biggest shake-up of Sunday trading laws in over 20 years. The government said the SNP's decision was "extraordinary" and "hypocritical".

The Conservative rebellion is being led by Christian MP David Burrowes who called for a "more sensible and palatable way forward".

"It is a fairly unholy alliance but it is not so much about what the SNP thinks but how the government will deal with the large concerns across party," he said. He defended the SNP's decision despite the fact the plans would not apply to Scotland because of the "domino effect" it could have on Scottish workers' pay.

Scotland already has relaxed Sunday trading laws but workers enjoy premium pay for working on a Sunday. The SNP has said it fears any change to current laws in England and Wales, which limit opening hours for large stores to six hours on a Sunday, would reduce pay for Scottish workers.

Burrowes' rebellion has gained the support of at least 23 Conservative MPs and could mean the plans, masterminded by Chancellor George Osborne, will be defeated only a week before the Budget.

A UK government source said: "It's disappointing and hypocritical of the SNP to be trying to deny people the freedoms to shop that are already available to those they represent in Scotland."

The SNP's decision came after Christian and Muslim leaders united to urge them to vote against the changes. In a public letter on Monday, leaders in the Catholic, Episcopal and Free Churches in Scotland were joined by the Muslim Council of Scotland as well as campaign groups including CARE, the Christian Institute and the Evangelical Alliance.

The joint letter argued the issue "most certainly is" a matter for Scotland and called on all Scottish MPs to "stand up for Scottish workers and Scotland" and vote against the proposals.