There isn't one.

Try and do a search on here for the subject- you will find quite a lot of info

Andy

It has been debated endlessly on the forum.

I think that there is a ‘legal’ answer in that employers can generally set their own rules, policies and procedures, that will form part of the contract of employment, and so long as these are not contrary to statute law or other ruling, then they will be 'enforceable' and ‘legal’ (unless subsequently successfully challenged).