Royal Mail insists it is continuing to give first class mail priority over second class at all times after it was reported that it had stopped doing so on Saturdays.
The Mail on Sunday said customers had been needlessly putting first class stamps on post that has been treated the same as second class - making millions for Royal Mail since the changes were "secretly introduced" in July.
The newspaper said it had seen a confidential document circulated to managers in mail centres, which says: "Second class traffic posted Friday and Saturday will be processed along with first class posted Saturday, using Saturday first class labels."
It adds: "We want to move to a standardised way of dispatching our second class Saturday-posted traffic and to also simplify and reduce cost in our network by removing under-utilised services.
"Moving the traffic forward to a Saturday dispatch and ceasing Monday's second class dispatch is the most viable and cost-effective option that also protects quality."
The newspaper said the move had made the company £4 million from customers needlessly putting first class stamps on post that had been treated the same as second class.
It claimed that around 40 million 46p first class stamps will have been used since July when a 36p second class stamp would have been just as quick, meaning there had been an unnecessary overpayment of £4 million.
However Royal Mail spokesman David Simpson said: "The Mail on Sunday is completely wrong to state that Royal Mail has 'stopped giving priority' to first class mail on Saturdays. The opposite is the truth. First class mail always has the priority over second class mail at all times.
"There has been no change at all in the service specification for first and second class mail. While we are committed to excellent service for all our mail, first class mail is our priority; we do everything possible to deliver it the next working day. The specification for second class mail is still to deliver it within three working days after posting.
"The UK is one of the few countries in Europe that provides a six-day postal service. That service is expensive to run and it has been losing significant amounts of money for some time."