It says up to 40pc of free range eggs tested fail the freshness test, and that the very definition of free range is wanting.

Choice says there's an urgent need for a clear and national description of free-range eggs after its investigation found the big producers have redefined the term to suit themselves.

The organisation says there is no legal definition of 'free-range' eggs and the Australian Egg Corporation's standards don't meet those recommended by animal welfare organisations.

"We need tighter regulation of the term free-range. Well over half the hens described as free-range are housed in huge sheds, may never go outside and their eggs may come off conveyor belts," Choice spokesman Christopher Zinn said.

While the European Union has enforceable standards, in Australia there are only voluntary standards set by the Free Range Egg and Poultry Association of Australia (FREPAA), Biological Farmers of Australia and animal welfare groups, including the RSPCA.

The Australian Egg Corporation, which represents 90pc of producers, has a quality-assurance scheme which includes free-range egg production but its standards allow more birds per square metre of shed or range area than others.

The scheme, called Egg Corp Assured, allows 14 birds per square metre in the shed, FREPAA standards only seven and EU guidelines no more than nine.

By comparison, cage systems, which supply more than two-thirds of the eggs sold in supermarkets, allow the space equivalent of 18 birds per square metre.

"Choice bought every brand of free-range egg we could find across much of the country and only one was independently certified by an agency apart from the Egg Corporation," Mr Zinn said.

"One of the larger producers said the other certifiers set 'unreasonable conditions'."

Choice has warned consumers buying eggs for animal welfare reasons that their best option is to buy certified organic eggs (check the labels) or go to smaller shops for eggs with independent free-range certification.

Of almost 700 free-range eggs tested, 36pc failed an internationally-recognised test for freshness, being described as "weak and watery".

Choice also wants to see eggs stamped with the date on which they were packed instead of unreliable best-before dates.