Allen Simpson, CEO of UKHospitality, which represents some of the restaurant brands who have withdrawn from the BCC, said despite the change the businesses are "committed to enhancing their ongoing work across welfare and the environment".
The eight businesses, owning or franchising 18 leading restaurant chains, have instead joined the industry-led Sustainable Chicken Forum (SCF).
Some of the brands include include Popeyes, Wingstop, and Wagamama.
The SCF said businesses "no longer believe the BCC is the right framework to drive the next phase of progress on welfare due to the requirement to source only slower-growing breeds".
It argued that farming slower-growing breeds produce more greenhouse gas emissions than farming faster-growing breeds.
UKHospitality said the SCF will focus on increasing welfare while limiting the environmental impact of chicken farming and keeping up with demand.
KFC in 2024 said it was rescinding its commitment to phase out buying fast-growing chicken, but the decision on Thursday withdrew from the Better Chicken Commitment entirely.
The BBC understands the famous chicken chain owned by Yum Brands buys about 4% of the UK chicken supply. An industry source said there is not yet a large enough supply of slower-growing poultry in the UK.
However, animal welfare group Anima International said the change was "about money and nothing else".
Its UK chief executive Connor Jackson said the companies' explanation for leaving is rubbish.
"Franken-chickens pose the most serious animal welfare problem in the UK," Jackson said.
He said that chicken farmers and poultry companies would produce more slower-growing, higher-welfare chickens according to orders.
"This is why the supply isn't there for them now: producers won't do it unless they have a contract in place because they are more expensive," he said.
Claire Williams, campaigns manager at The Humane League UK, said it was clear to her animal welfare group why the chicken outlets had switched membership.
"Major food companies, with the combined worth of many billions of pounds, have decided that their profit margins cannot be threatened," Williams said.
"The Better Chicken Commitment was designed by scientists to help animals - the Sustainable Chicken Forum is a welfare-washing, PR-stunt designed to deflect criticism, and let these companies claim they are doing enough".
Richard Griffiths, head of the British Poultry Council, which represents poultry breeders and processors, said the news was "most welcome".
He said that at a time of higher production costs and slow planning approvals the industry did not need added costs.
Although there has been a surge in the number of takeaway chicken shops in the UK in recent years, British supermarkets remain the major purchasers of poultry.
Retailers such as M&S and Waitrose and cafe chains Pret and Greggs remain members of the BCC.
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