In a landmark 2005 House of Lords case (commonly referred to as the Wilkinson case), the UK courts considered a significant number of Extra Statutory Concessions (ESCs) published by HMRC. The decision by the House of Lords, questioned the ability of HMRC
to make use of certain administrative discretions (such as an ESC).

An ESC is a concession granted by HMRC that usually reduces a taxpayer’s tax liability to a level less than they would usually pay based on a strict reading of the law. The majority of the ESCs introduced by HMRC deal with anomalies in the legislation, which
under certain specific situations create unintended consequences for taxpayers and which are difficult to remedy from a legislative effect.

HMRC has recently published an updated version of the guide that details the ESCs previously operated by Inland Revenue that were still in place on 6 April 2018. A similar list of ESCs previously operated by HM Customs & Excise, can be found in Notice 48.
The number of ESCs still in use has reduced with many becoming obsolete. 

Source: New feed