David Cameron responded to a question at Prime Minister's Questions from a Labour MP who said plans to let big shops open longer were "causing great concern".
The PM said all the evidence shows the step would be welcomed by customers and create more jobs.
He argued it was "right" to bring forward the plans, devolving the decision to local authorities, adding the Government would be introducing new protections for all workers on Sundays.
Rejecting criticism from the Church and several Christian organisations Mr Cameron urged MPs to "look carefully at this idea".
Labour's Bill Esterson asked why Mr Cameron changed his mind on the issue.
"The proposed changes to Sunday trading are causing great concern to many retailers, shop workers, to their families, to faith groups and to all who want to keep Sunday special," said the MP for Sefton Central.
"Yet before the election the Prime Minister said he had no plans to change Sunday trading laws. When did he change his mind, or was it always his plan to scrap this great British compromise as soon as the election was safely out of the way?"
Mr Cameron replied: "Well I thought it was right to bring forward these proposals because they are genuinely new proposals, new in that we are devolving to local authorities to make this decision, and secondly and crucially and I'm sure honourable members opposite will be interested in this, is that we will be introducing new protections not only for new workers on Sundays but for all workers on Sundays.
"And so I think the House should look carefully at this idea, not least because our constituents are able to shop online all day, every day including Sunday.
"All the evidence shows this will be welcomed by customers, will create more jobs and I think we have nothing to be scared of, of moving into this new arrangement."
The Government is pressing ahead with plans to allow local councils to extend Sunday trading hours.
Amendments to the Enterprise Bill will be introduced to enable the change in hours to be made in the autumn.
Ministers said that local retailers will have the flexibility to adjust hours so they can compete for trade.