David Cameron has been meeting Conservative backbenchers as the Government battles to see off a rebellion which could scupper its plans to extend Sunday trading hours.
With up to 30 Tory rebels expected to combine with Labour and the Scottish National Party to strike the proposals out of the Enterprise Bill, the Government offered a last-minute concession which would see the changes initially limited to 12 pilot areas in England and Wales.
MPs were denied the opportunity to debate the 11th-hour compromise, after Speaker John Bercow declined to provide Commons time. But communities minister Brandon Lewis assured them that the pilot scheme will instead be tabled in the House of Lords if they reject the rebel amendment.
Meanwhile, the Business Department released the results of an official assessment of the potential impact of extended Sunday trading hours, which suggested liberalisation could benefit the UK economy by an estimated £1.5 billion or more over 10 years.
Individual households could expect to benefit by the equivalent of £29 as retailers pass on savings generated by the ability to maximise the use of large stores, the department suggested.
Setting out details of the latest concession, Mr Lewis said the Government was ready to ditch its plan to extend powers to set Sunday opening hours to councils nationwide on the first day of the new regime.
Instead, it will invite local authorities to apply for places on the year-long pilot scheme. Twelve locations, which are nominated locally and are "geographically, economically and demographically diverse" will be selected to take part in a study to determine the impact of liberalisation.
The prospect of a damaging Commons defeat was raised after the SNP said it would vote with Tory rebels to block the Sunday trading measure.
The move - after the nationalists had previously indicated they would abstain - infuriated ministers, with one Government source branding their action as "disappointing and hypocritical".
And Mr Cameron joined last-ditch efforts to stave off defeat by speaking personally to Tory MPs with concerns about the impact of longer Sunday opening hours on family life and the viability of smaller shops.
The PM's spokeswoman said that Mr Cameron was "engaging" as part of the Government's work to ensure its business gets through Parliament.
With up to 30 Tory MPs reportedly planning to defy the whips and vote against the plan - and another 20 said to be ready to abstain - the parliamentary arithmetic appears to be stacked against the Government which has a working majority of just 17.
Defeat would be a particularly bitter blow for Chancellor George Osborne, who first announced his plan in his summer Budget following last year's general election victory.
He said that allowing councils in England and Wales to decide whether larger stores should be able to stay open for longer than the current maximum of six hours could help "struggling" high streets to compete with online retailers.
However Labour and the unions have argued that it would lead to an erosion of shop workers' pay and conditions across the UK while the Church of England has also voiced concern about the impact of the changes.