HMRC have published The Income Tax (Pay As You Earn) (Amendment) Regulations 2011 no 729 to amend the Income Tax (Pay as You Earn) Regulations 2003 (SI 2003/2682) to define the additional rate of income tax, as well as the 0T and additional rate income tax codes.
They require employers to deduct income tax using the 0T rate code instead of the basic rate tax code in specified circumstances.