Fast-growing chickens which reach their slaughter weight quicker than other types are the equivalent of a 28-stone (177.7kg) three-year-old, the Court of Appeal has heard.

Animal welfare charity, The Humane League UK, previously brought legal action against then Secretary of State for Environment, Food and Rural Affairs Therese Coffey, arguing she had misinterpreted a key section of animal welfare law concerning so-called “Frankenchickens”.

These are chickens which can reach a slaughter weight of more than 2.0kg in five-six weeks as a result of genetic selection when reared in an indoor commercial environment.

Last year, the High Court was told that the fast-growing breeds experience significantly worse welfare detriment than slower-growing breeds, risking leg deformities, organ failure and chronic pain.

The Humane League court case
Animal welfare supporters outside the Royal Courts Of Justice in London (Ben Whitley/PA)

The Humane League UK argued this meant Ms Coffey had therefore misinterpreted part of animal welfare regulations.

In May 2023, Sir Ross Cranston dismissed the legal challenge, saying Ms Coffey had not “positively authorised or approved unlawful conduct by others” so as to be in breach of any public law duty.

However, on Wednesday, judges at the Court of Appeal in London heard a bid to overturn Sir Ross’s ruling by the charity.

The RSPCA is taking part in the appeal, telling the court that it supports the Humane League’s case.

In written submissions, Nick Armstrong KC said: “The RSPCA considers that the suffering experienced by fast-growing breeds of broiler chickens now represents the most important animal welfare issue in the world.”

The barrister said that the scale of harm caused to 1.1 billion affected chickens was “enormous”, adding: “It cannot be minimised.”

He continued: “Perhaps the best way of explaining the impact is to point out that the human equivalent would be a 28-stone three-year-old.”

Edward Brown KC, for the Humane League UK, said in written submissions that the systems for checking welfare conditions for fast-growing chickens are unlawful.

The barrister said they are “incapable of identifying harm that is required to be identified … save where the levels of detriment are ‘exceptionally high’ or the mortality rate ‘unusually’ high”.

“Those thresholds are set so high that significant welfare detriment will never be identified and/or reported,” Mr Brown continued.

The Department for Environment Food and Rural Affairs (Defra) is opposing the appeal, with its lawyers telling the court that the Secretary of State has not acted unreasonably.

Richard Turney KC, for the department, said in written submissions that Defra does not accept there is a scientific consensus that fast-growing chicken breeds cannot be kept without an impact on their welfare.

He continued: “Whilst the Secretary of State accepts that there can be a higher risk of some welfare issues with fast-growing breeds of meat chicken, that does not mean that they will have unacceptably poor welfare in all circumstances.”

Addressing the systems for checking welfare conditions, Mr Turney added: “The provision requires monitoring to identify poor welfare conditions, not to identify individual incidence of conditions which might be consistent with welfare issues.”

The National Farmers’ Union is also opposing the appeal, with its barrister Malcolm Birdling telling the court in written submissions that the Humane League UK “seeks to side-step the consequences” of the lawful decisions made by Defra.

The hearing before Lord Justice Underhill, Lord Justice Males and Lord Justice Snowden is due to conclude on Thursday with a decision expected at a later date.