Attempting to prevent a witness from giving evidence or to influence the evidence he gives. Making improper threats against witnesses may amount to the common-law offence of perverting the course of justice (R v Kellett  QB 372); persuading a witness to tell a lie constitutes – in addition to this – the offence of subornation of perjury. It is also perverting the course of justice to put pressure upon a witness to give evidence or to pay him money to testify in a particular way. Sometimes interfering with witnesses may also amount to contempt of court. There is also a separate common-law offence of tampering with witnesses when one uses threats to persuade them not to give evidence. See also intimidation.