Nickel has been shown to be the most common cause of skin contact allergy in Europe. European Directive 94/27/EC was made UK Law in 2000 and specifies the upper limit for nickel release in articles which have direct and prolonged contact with the skin - such as Jewellery, fashion accessories, and metal adornments for fashion.
The ‘Nickel Directive’ 94/27/EC does not ban the use of nickel per se. For the majority of items it only restricts the release of nickel ions from the item. Alloys that contain nickel that don't corrode, i.e. some stainless steels, don't release nickel ions and so may be used.