This excerpt taken from the BP 6-K filed Sep 7, 2005.
Deferred tax adjustments
Under UK GAAP, deferred tax is provided on timing differences, whereas IFRS requires provision to be made for temporary differences between carrying values and the related tax base. As a result, deferred tax needs to be recognized under IFRS in respect of a number of differences for which no deferred tax was recognized under UK GAAP. The major areas affected by this are described below.