George Osborne has defended his plan to lift Sunday trading restrictions this summer as angry union bosses prepared for urgent talks with ministers about the surprise move.
Emergency legislation will be promised by the Chancellor in Wednesday's Budget to allow supermarkets and other big stores to open all day during the London Olympics and Paralympics.
He insisted it was a temporary measure for eight weeks to maximise the economic benefits of the sporting spectacle which will attract crowds of visitors from across the globe.
The option of a permanent relaxation of the rules remains under consideration by the Treasury however, sparking claims the Games were being used as a smokescreen for full-scale deregulation.
Shopworkers' union Usdaw said there was no evidence that retailers or shoppers wanted the change which it said would do little to boost spending but could wreck the family life of many workers.
General secretary John Hannett said he would meet with Culture Secretary Jeremy Hunt on Tuesday to discuss their concerns and complained about the lack of consultation.
Ministers want to push a law change through Parliament within just the next 10 days but could face a fight, with Labour and several Tory MPs already expressing serious concerns.
Under the Sunday Trading Act 1994, large shops over 280 square metres in England and Wales are restricted to six hours' continuous trading between 10am and 6pm on Sundays.
One consequence would be that the three biggest souvenir shops at the Olympic village itself in Stratford, east London would be forced to close their doors to spectators.
Mr Osborne said it would be a "great shame" not to use the opportunity presented by the Games.
"We have got the whole world coming to London and the rest of the country," he told BBC1's Andrew Marr Show.
"It would be a great shame, particularly when some of the big Olympic events are on a Sunday, if the country had a closed-for-business sign on it."
But shadow chancellor Ed Balls warned him against "breaking up centuries of tradition".
Demanding a proper consultation, he said: "Today, on Mothering Sunday, there are mums at home with their kids because Sunday trading (laws) means they can have the morning off.
"We should be very careful about breaking Sunday trading."
Usdaw said a Government-ordered consultation only last year had found no widespread support either among shoppers or retailers for a long-term change which would be "vehemently opposed" by its members.
"Deregulation would do little to stimulate growth or create jobs but would have a very detrimental impact on the lives of millions of shopworkers and their families," Mr Hannett said.
"Any change would fly totally in the face of the Government's commitment to be family-friendly."
He ridiculed the idea that a country that allowed shops to open 150 hours per week could be seen as "closed".
He said: "There is understandable suspicion that the government is trying to use the London Olympics as cover for its wider deregulation agenda.
"Shopworkers are entitled to expect some respect from the government and for their views to be heard and taken into account before any decisions of this importance are made."
Campaign group Keep Sunday Special said the change was "a cover for creeping deregulation" and urged MPs to oppose the legislation.
"Research by the National Centre for Social Research has shown that Sunday working has a detrimental impact on fathers' time with their children, especially on playing, reading and teaching," it said in a statement.
"When did shopping become an Olympic sport? Why are the Olympics deemed to be a special case?"
"It would be shameful indeed if Parliament allowed a change to be pushed through in the context of the Budget, especially as there was consultation only last year which showed conclusively that there was no appetite to change the law."
Among Tory MPs raising concerns was Therese Coffey who said any temporary relaxation should apply only to shops at the Olympic venues and should not be used as a route to a permanent change.
She expressed doubt that it would create the jobs that supporters claimed and warned that it could instead put huge pressure on struggling independent retailers and disrupt the limited time for families to be together.
"What we have now provides a reasonable balance," she said.
But it was warmly welcomed by party colleague Mark Menzies who launched a similar Commons bid last year.
The MP for Fylde, whose professional background is in retail, said he had been told an estimated 1,700 part-time jobs could be created as a result at the giant new shopping centre near the Olympic park.
He insisted he had "no desire at this stage" to see the looser rules applied beyond August.