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By section 1(1) of the Perjury Act 1911, perjury is committed when:-
a lawfully sworn witness
The offence is triable only on indictment and carries a maximum penalty of seven years' imprisonment and/or a fine.
A conviction cannot be obtained solely on the evidence of a single witness as to the falsity of any statement. There must, by virtue of section 13 Perjury Act 1991, be some other evidence of the falsity of the statement, for example, a letter or account written by the defendant contradicting his sworn evidence is sufficient if supported by a single witness.
Perjury is regarded as "one of the most serious offences on the criminal calendar because it wholly undermines the whole basis of the administration of justice":- Chapman J in (R v Warne(1980) 2 Cr. App.R. (S) 42). It is regarded as serious whether it is committed in the context of a minor case, for example a car passenger who falsely states that the driver did not jump a red light as alleged, or a serious case, for example a false alibi witness in a bank robbery case.
In most cases, an offence of perjury will also amount to perverting the course of justice. If the perjury is the sole or principal act, then it will be normal to charge perjury. If the perjury is part of a much more significant series of acts aimed at perverting justice, then a charge of perverting the course of justice would be more appropriate.
A charge of perverting the course of justice cannot be brought simply to avoid the requirements of corroboration of the falsity of the evidence as required by s.13: (Tsang Ping Nam v R 74 Cr. App. R. 139 PC).
Perjury by a prosecution witness
Proceedings against a prosecution witness for perjury will depend on an assessment of the material effect of the perjured evidence. If a wrongful conviction is believed to have occurred because of the perjured evidence, a prosecution should follow, unless there are exceptional circumstances. If the witness has lied to protect his or her own interests rather than with an intent to pervert the course of justice, a prosecution may be unnecessary.
Perjury by a defendant
If a defendant is convicted despite giving perjured evidence, the decision to prosecute must take note of the sentence imposed for the original offence. If you think a conviction for perjury is unlikely to result in a substantial increase in sentence, then the public interest probably does not require a prosecution.
Consider also the possible consequences to the original conviction of an acquittal of the defendant on a charge of perjury arising out of the earlier proceedings. You should, therefore, be satisfied that the evidence of perjury is exceptionally strong before instituting proceedings.
Evidence of premeditation is an important factor in coming to a decision on whether or not to prosecute. If the defendant's lies have been planned before the hearing as opposed to arising on the spur of the moment during cross-examination, the public interest in prosecuting will be stronger.
Where a defendant is acquitted, wholly or partly because of false evidence given by him or her, a prosecution for perjury might be appropriate. Where there is clear evidence of perjury, which emerges after the trial, and which goes to the heart of the issues raised at the trial, a prosecution for perjury may be appropriate. A prosecution should not be brought, however, where it may give the appearance that the prosecution is seeking to go behind the earlier acquittal: see dicta by Lord Hailsham L.C. in (D.P.P. v Humphrys  AC).
Perjury by a defence witness
The decision to prosecute a defence witness for perjury partly depends on whether the defendant in the earlier trial was convicted:
If the defendant was convicted,
and there is no clear evidence of collusion, a prosecution would not
usually be appropriate;
Offences akin to perjury
There are a number of offences akin to perjury in the perjury act 1911 which, should be considered, including:-
false statements on oath
made otherwise than in a judicial proceeding: s.2;
These offences may overlap with other criminal offences, such as forgery or deception. The more flagrant the breach of the appropriate section of this Act, the more likely it will be that the defendant should be prosecuted for an offence under the Act as well as any other offences that arise.
Where the false evidence is tendered in written form under:
Section 9 Criminal Justice
Act 1967, an offence is committed under section 89 of that Act