RSPCA New South Wales’s vice-president has left the animal welfare group, saying he is “sincerely ashamed” of the standards for RSPCA-certified chicken.

Mark Steele, a senior counsel who was a volunteer member of the board from 2021 also served as a non-executive director of RSPCA Australia until March, issued a statement on LinkedIn outlining his decision to leave the group’s NSW branch for good at the end of September.

“The RSPCA does much fine work,” he wrote. “But there is one thing I have long been ashamed to be associated with: the use of the RSPCA’s trusted brand, through the RSPCA Certified scheme, to certify the welfare of 97% of Australia meat chicken.”

Chicken accounts for almost half of all meat eaten in Australia. Consumption has doubled in the past 30 years, and Australians now eat an average of about 55kg of chicken meat per person per year.

Almost all of the 800 million chickens slaughtered for meat in Australia each year are from one of two fast-growing breeds, which reach slaughter weight at between four and six weeks of age. Animal welfare experts say this rapid growth can stress the animals’ bodies.

Most chicken produced in Australia is also RSPCA Certified, a badge which signals to consumers that the product meets higher welfare standards.

“In my view, a shopper who sees that RSPCA logo would naturally assume – and ought to be entitled to assume – that the bird lived a good life and died a humane death,” Steele wrote. “In my opinion, a life like that cannot fairly be called good; which is why I don’t buy this chicken.”

Jed Goodfellow, the director of policy and government relations at Australian Alliance for Animals, said meat chickens grown that quickly can experience a range of issues, including cardiovascular, skeletal and dermatological problems “as their biology is pushed to the limits”.

“Healthier slower growing breeds are now being used in many parts of the world, particularly Europe … It may cost slightly more, but as their experience shows, the increase is not prohibitive, as European consumers increasingly opt for the healthier option,” he said.

Steele’s post comes just weeks after the RSPCA upgraded its animal welfare standards for chicken meat, which must be enacted by 1 January 2031 for producers to maintain their certification. Those changes include lower stocking densities, variable light intensity in sheds, and the phasing out of a slaughtering method called electrical water bath stunning.

But the new standards do not include phasing out fast-growing breeds. Instead, the organisation announced a new category of “enhanced” welfare standards which encourage producers to switch to slower-growing chicken breeds. The enhanced category does not yet have a label for consumers to identify, and those standards are voluntary.

Steele said the new standards were a “small step forward” but criticised both the opt-in encouragement to adopt slower-growing breeds and the lack of an end date to phase out fast-growing breeds.

He said consumers would not recognise the welfare issue with fast-growing breeds “while the most trusted name in Australian animal welfare sits on the packaging, answering the question before the shopper has even thought to ask it”.

“Until it does, I will remain sincerely ashamed that the name of an organisation I otherwise love and respect appears on this chicken.”

Last month, the RSPCA’s senior scientific officer, Melina Tensen, told Guardian Australia the organisation would ultimately like to see slower-growing breeds replace fast-growing breeds.

“We definitely encourage consumers to be vocal about supporting this transition … because every dollar you spend is a message to the retailer,” she said.