All those eggs you bought thinking they were kosher may not quite be as they seem, writes Rosa Docherty. According to the Jewish Vegetarian Society, it only needs a hen to be later killed the wrong way for its eggs to lose kashrut status.

The organisation says that for an egg to be kosher it needs to be from a chicken that has suffered needless harm neither in life nor death.

That means that those gassed or killed by means other than shechita could effectively lose any right to kosher status long after their eggs have been bought and eaten.

The JVS says many stores are also breaking rules by selling eggs produced by birds kept in cramped conditions. “Whether battery or free-range, hens are gassed or killed by other means after they have been used to make eggs,” says director Lara Smallman. “This means eggs sold on the high street, including in kosher shops, directly contravene tza’ar ba’alei chayim, the prohibition against causing unnecessary pain.”

The JVS has gained support from rabbis for its stance. Rabbi Natan Levy from the Board of Deputies said: “I’ve signed my family up to get the eggs from JVS because they are proper kosher eggs that follow the laws. Anything that comes from a treif animal is treif. If the community took this seriously we’d have to have a huge overhaul of how we currently source our eggs and what kosher shops offer.”

Simon Rocker writes: If a well-treated chicken was subsequently inhumanely killed, that would not retrospectively render the egg treif. The JVS’s claim that only eggs from chickens reared outdoors and later humanely slaughtered are fit for Jewish consumption is an attempt to stretch the definition of kosher.

The London Board for Shechita does not use battery-farmed chickens but will take birds reared in sheds where “there is room to run”.

Some Orthodox rabbis want to see more rigorous standards of animal care. Rabbi Levy argues along the following lines: it is forbidden to eat a chicken with a broken wing, according to the laws of kashrut. Birds raised in restrictive conditions have a higher likelihood of injury. Eggs with a strong chance of deriving from injured birds should therefore be treated as unkosher and avoided.