The CAA (GR16) also require with effect from 1 April 1988, all braked wheels of retractable landing gear units on aeroplanes above 5700 kgs will be required to have tyres inflated with Nitrogen, or other suitable inert gas, and maintained such as to limit the Oxygen content of the compressed gases to not greater than 5% by volume.
http://www.caa.co.uk/docs/33/CAP747.PDF