Sir Brian May has resigned as the RSPCA’s vice president over what he called “appalling” animal welfare standards in farms certified under its scheme.

The certified RSPCA Assured label, which indicates that meat, fish, eggs and dairy products have been produced to strict standards which exceed the UK’s legal requirements, is being reviewed by the animal welfare organisation.

Almost 4,000 farms participate in the scheme, which means they can use the label to inform supermarket shoppers of their superior animal welfare standards.

Queen guitarist Sir Brian, who has long campaigned on animal welfare rights, and is a co-founder of the Save Me Trust, which campaigns for the rights of foxes and badgers, shared his letter of resignation on Instagram.

The letter read: “It is with profound sadness and not without massive soul-searching that today I have to offer my resignation as a vice-president of the RSPCA.

“You have kept me informed through the Save Me Trust of complaints that have been levelled in recent months at the RSPCA over appallingly bad standards of animal welfare in member farms of the RSPCA Assured scheme.

“I have understood that the RSPCA needed time to evaluate the evidence and make decisions on action to be taken. But as more and more damning evidence comes to light, I find the RSPCA’s response completely inadequate.”

He added that as the supervision of the scheme has “failed”, it needs to be dismantled.

Sir Brian also said: “I am in full acknowledgment of the wonderful work that the RSPCA has done over the years, and I am heartbroken to be withdrawing my support from the day-to-day work performed by so many heroes on the ground every day.”

He added that he understood the “danger of weakening the RSPCA, and playing into the hands of the pro-cruelty organisations which the RSPCA has been instrumental in prosecuting”.

However, he “cannot in all conscience remain a figurehead of the organisation while the RSCPA avoids its responsibility to put its house in order”.

Allegations include overcrowding, poor hygiene, and in some cases, physical abuse of livestock by farm workers.

TV presenter Chris Packham also called for the scheme to be suspended.

The broadcaster, who is president of the RSPCA, and former Green MP Caroline Lucas, a vice president, released a joint statement saying they “understand, sympathise with and respect his decision”, adding that the issues he has resigned over are “of grave concern to us”.

“We have been in constant talks with senior management at the RSPCA over reform or scrapping their Assured schemes,” they said.

In terms of the RSPCA’s review of its schemes, they said: “We will be considering what action to take as soon as we have seen this overdue report. It would be dishonourable and disingenuous to take further action before it has been shared with us.”

They pair added they have “other serious concerns” about the RSPCA’s attitude to animal agriculture.

An RSPCA spokeswoman said it has been “very proud to have Brian May, a passionate campaigner for animals, as our vice president and we share his desire to create a better world for all animals”, but added that it has “different views from Brian on how best to approach this complex challenge”.

She also said they “respect his views and understand his decision” before adding: “We would like to thank him for everything he has done for the RSPCA, our staff, volunteers, supporters and, of course, the animals, during his time as vice president.

“His ongoing and devoted work campaigning on issues such as the badger cull and hunting have been invaluable for all animals and we look forward to speaking up on these issues with him in the future.”

The spokeswoman also called farming “hard, and farmed animal welfare is even harder”, and the “reality is 94% of people currently choose to eat meat”.

“The rapid growth of low welfare industrial farming is the biggest welfare issue facing animals,” she added.

“There are billions of animals being farmed right now in appalling conditions, with no regard for their welfare.

“There is currently no specific legal protection for salmon, trout, turkeys, dairy cattle, beef cattle and sheep, and without our standards many millions of farmed animals would live worse lives.

“But we want to give our supporters, partners and the public confidence that RSPCA Assured is consistently delivering better welfare than standard farming practices.

“So, we launched an independent review of RSPCA Assured, which has been carried out over several months, including unannounced visits to more than 200 members of the scheme. Once we have analysed our findings, we will take any robust action necessary.”