The Prime Minister reassured campaigners as recently as April that the Conservatives had no “current plans” to relax Sunday trading laws.

In a letter to the Keep Sunday Special campaign group, a spokeswoman for David Cameron said the current system provided “a reasonable balance between those who wish to see more opportunity to shop in large stores on a Sunday, and those who would like to see further restrictions”.

“I hope you find this reassuring,” concluded the letter, written on 20 April.

Chancellor George Osborne will announce plans in the Budget tomorrow to devolve powers on Sunday Trading rules to local authorities and elected mayors.

David Cameron’s official spokeswoman said today: “The Prime Minister thinks that the measures that are being set out today, about how we devolve the decision to local authorities on this issue, is the right thing to do.

“This is about enabling communities to have more choice about what is right for their high streets and allowing councils and city mayors to take decisions based on the priorities that they know best.”

Osborne said: “Even two decades on from the introduction of the Sunday Trading Act, it is clear that there is still a growing appetite for shopping on a Sunday. There is some evidence that transactions for Sunday shopping are actually growing faster than those for Saturday.

“The rise of online shopping, which people can do round the clock, also means more retailers want to be able to compete by opening for longer at the weekend. But this won’t be right for every area, so I want to devolve the power to make this decision to mayors and local authorities.”

ACS research conducted in February 2015 by ComRes showed that 76% of the public support existing Sunday trading regulations. Of those who were in favour of change, 60% called for greater restrictions on trading hours.

During the temporary removal of Sunday Trading rules during the Olympics, sales actually declined by 0.4% overall, with some smaller retailers reporting a sales decline of up to 20%, according to the ACS.