All chicken eggs that are sold must be stamped with a code indicating the production method, country of origin (two letters) and farm (five digits), for example 1UK12345.

The first number refers to the production method. As with the labels for farming techniques for table birds, the maximum 'stocking density' is stipulated.
0 - Organic
1 - Free range: birds are housed in a barn but have continuous daytime access to open runs that are mainly covered with vegetation.
2 - Barn: birds are housed in a barn with a series of perches and feeders at different levels.
3 - Caged: birds are housed in tiers of cages which have sloping mesh floors so that the eggs roll backwards out of the reach of the birds to await collection. Increasingly the cages are 'enriched' as legislation dictates that they must all be thus by 2012.

In addition, some eggs will have a quality mark and/or a best before date stamped on them.

The Lion Quality mark assures the purchaser that the eggs stamped with this mark come from British hens that have been vaccinated against salmonella and have been produced to high standards of hygiene and animal welfare. The mark is stamped on the eggs and egg boxes. A best before date is also stamped on the eggs.

'Laid in Britain' is a quality and food safety assurance scheme devised for independent egg producers, who pack and mark their own produce, to supply retailers and caterers on a local and regional basis. The logo appears on the eggs boxes but not on the eggs themselves.

Duck and goose eggs do not have to be stamped with the ID mark.