Customers, including BHS, EDF Energy, National Grid and NS&I, have sent a total of more than 30m pieces under the scheme.

Robert Keitch, chief of membership and brand at the Direct Mail Association, which is backing the scheme, said: "We're really impressed. Royal Mail has worked incredibly hard on this. It put in a lot of work in an area that might be considered oblique and deemed not that important.

"It's reassuring that there are so many brands out there that think beyond the obvious."

The scheme, which applies to MailSort tariffs, offers two tiers of criteria to qualify for discounts of 2% or 4.7% and is compliant with the PAS 2020 BSI standard.

The entry-level criteria include using certified paper stocks, responsible data practices, recyclability of paper elements and encouraging this through logos on mail pieces.

The intermediate level adds sourcing print production through ISO 14001-certified printers and avoiding UV varnishing or rubber-based glues, as well as clear and effective unsubscribe processes.

An internal Royal Mail team currently audits Sustainable Mail customers' adherence to these requirements, although the reliance on this will be reduced as participants become PAS 2020 certified.

Matthew Neilson, head of environmental solutions at Royal Mail, said: "Adopting the specification will also increase the likelihood of the mailing being recycled with consumers claiming it would help them to recycle over 80% of their existing direct mail.

"This means that the more customers embrace Sustainable Mail, the more likely the industry will achieve its 55% recycling target by the end of the year.

"Sustainable Mail is focusing on direct mail and will support the introduction of PAS 2020. We are also exploring other environmental opportunities for our mail services."