The common-law offence of escaping from lawful custody. The custody may be in prison or a police station, or even in the open air. The escaper need not have been charged with any offence, provided his detention is lawful (e.g. he may be detained to provide a specimen of breath). Nor is it necessary for him to commit any act of breaking out. It is also an offence to help the escape of a prisoner and to permit a prisoner who is detained in relation to a criminal matter to escape. If someone actually breaks out of a building in which he is lawfully confined he commits a separate offence of prison breaking. Case: R v Dhillon  1 WLR 1535.
From: escape in A Dictionary of Law »