EU legislation, set to be brought into the UK next year, allows producers to rear chickens at a maximum density of 33kg of chicken/sq m. It also permits up to 39kg/sq m and 42kg/sq m if additional welfare guarantees are met.
Defra is currently considering whether to implement the legislation as it is, or whether to prevent producers from stocking to the higher limits.
The RSPCA launched a Quash the Squash campaign this week to persuade Defra not to allow the proposed upper limit. It claims the 42kg/sq m limit would make it harder for chickens to perform basic actions such as walking and flapping their wings.
"Implementing the new provisions could have a significant impact on mortality rates and skin and leg problems," said senior farm animal scientist Dr Marc Cooper.
However, UK producers have warned that if they were not allowed the higher densities, they would lose out to producers from other countries that did allow them.
Domestic suppliers should not be denied the opportunity of producing to 42kg/sq m if that market were available to EU rivals, said British Poultry Council chief executive Peter Bradnock and NFU chief poultry adviser Robert Newberry.
"If the consumer agrees with the RSPCA's sentiment then they will buy from [less densely stocked] systems," said Newberry.
A spokesman for 2 Sisters stressed higher stocking densities needn't mean lower welfare if they were combined with good facilities and management.
There are currently no stocking density limits in the UK, although producers under the Red Tractor scheme generally stock at 38kg/sq m and Freedom Food suppliers stock at 30kg/sq m.
Defra is expected to make a decision regarding implementation shortly.