It is a criminal offence to be in charge of or to drive a motor car when under the influence of excess alcohol.
The term “in charge” is capable of a much wider interpretation than driving and for example may include simply walking towards your vehicle with an intention to drive it!
In order to constitute an offence the level of alcohol in the blood must exceed 35 micrograms of alcohol in 100 millilitres of breath (or 80 micrograms in 100 millilitres of blood). As a matter of policy the CPS rarely prosecute unless the figure exceeds 40 micrograms in breath.
The prosecution are entitled to do an “add back” calculation arguing that the alcohol/blood ratio was “x” at the time of the test and it would have been so much more at the moment of arrest – on the basis that the levels deplete at a recognised rate per hour.
The prosecution will usually seek to prove their case by the use of samples – either a breath testing machine or by tests of either blood or urine. A challenge based upon efficacy of these tests or operator error is possible but, far from easy.
If a police officer has reasonable cause to believe that a driver has “alcohol in his body” he may require the driver to submit to a breath test. Failure to provide this “without reasonable excuse” is an offence in its own right.
Upon conviction the offence of driving or being in charge of a vehicle under the influence of alcohol may include a fine, imprisonment [generally reserved for serial offenders or those driving with a breath/alcohol level in excess of 100 micrograms of alcohol per 100 millilitres of breath]. Disqualification from driving and the amount of penalty points are mandatory except where there are “special reasons” – see separate briefing note.