A government compromise (pilot schemes) designed to stave off defeat was not tabled in time for MPs to vote on it, but Communities Minister Brandon Lewis said the government would amend its own Enterprise Bill later in the House of Lords.
The SNP's decision to vote against the Bill, even though Sunday trading is devolved to Scotland, has prompted accusations of hypocrisy and increased the prospect of a government defeat.
More than 20 Conservative MPs, including Byron Davies and David Jones, have signed a 'rebel' amendment  to the Bill.
Labour MPs appear to be wholly united against the Bill, which would devolve the power over Sunday trading hours to local councils rather than automatically extend shopping hours.
Welsh Liberal Democrat Mark Williams is minded to vote against (the Lib Dems have a free vote) but is listening to the debate before making a final decision.
The three Plaid Cymru MPs will vote against giving councils the new powers, arguing that the decision should be devolved to the National Assembly for Wales.
That option isn't on the table today, so Plaid will effectively vote for the status quo - for the power to remain in London rather than in town halls across Wales.
Plaid's parliamentary leader Hywel Williams said: "Had the Wales Office been a bit more muscular they might have been able to include powers over Sunday trading in the Wales Bill - and we would have supported that.
"They are devolving the power to the London (regional) assembly but not to the Welsh (national) assembly
"We are not in favour of the government here dealing directly with matters to do with our local authorities - that's a matter for our government in Cardiff as far as we're concerned."
UPDATE: The government lost the key division by 317 votes to 286.
The debate over Sunday trading hours is proving rather fractious with plenty of "blue-on-blue" action as Conservative MPs wonder publicly why it wasn't in their manifesto for last year's general election.