The rates, which are used to repay employees tax-free for fuel used on business mileage, apply to all journeys on or after 1 September 2012 until further notice.
For one month from the date of change employers may use either the previous or new current rates as they choose.
Employers may therefore make or require supplementary payments if they so wish, but are under no obligation to do either.
Rates are reviewed quarterly. The full list is available here