High Court judges will hear an appeal later that could change the way the nation's favourite meat is produced.
Ninety per cent of all supermarket chickens in the UK are a fast-growing breed, genetically selected for their rapid growth rate, according to the RSPCA.
The Humane League UK (THLUK) is challenging a decision by the court in 2023 that ruled the Department for Environment, Food and Rural Affairs (Defra) was not breaking the law by allowing their continued use.
THLUK says the birds, called 'Frankenchickens' by campaigners, are illegal because they endure "horrific suffering" due to their genetic selection. Defra is unable to comment due to legal proceedings.
Liam Hodgson, from THLUK, who was previously employed as a quality manager for a large poultry producer in Lincolnshire, said he found it "frustrating" trying to improve welfare standards when working in the industry.
He explained the birds are referred to as 'Frankenchickens' because they have been genetically selected over decades to produce as much meat as possible, with as little feed as possible, in the shortest time.
They reach slaughter weight within 34 to 36 days, with a growth speed said to be 12 weeks faster than it was 50 years ago.
Mr Hodgson said that he witnessed a wide range of health and welfare issues due to the speed of growth, "including lameness, hip dislocations, heart attacks and burns from laying in their own urine and faeces".
Undercover footage recorded in 2023 showed the condition of birds in a legally compliant poultry unit.
Kate Parkes, from the RSPCA, is supporting the judges by acting as an intervener - providing "hard scientific facts" to assist their decision-making.
Ms Parkes told the BBC that it was an "eye opener" inspecting farms that produce poultry from so-called "frankenchickens".
She said: "The scale of the suffering involved with meat chickens is huge - we are talking nearly a billion chickens every single year who live short, brutal lives with serious health and welfare issues which could so easily be avoided.
"A standard supermarket chicken grows so fast that if we were to genetically breed humans in the same way, it would be the equivalent of a morbidly obese 28-stone three-year-old - a stark image that really hits home why these chickens have been dubbed Frankenchickens."
The RSPCA's position is that fast-growing breeds should be phased out and replaced with a slower-growing alternative.
Following the decision by judge Sir Ross Cranston in May 2023 to dismiss the original case by THLUK, Defra said the government welcomed the judgement and added: "Farm animals are protected by robust animal health and welfare laws, which include detailed requirements on how they should be kept.
"We will continue to work with the farming sector to maintain and enhance our high standards."
The appeal is due to take up to two days at the High Court in London.