Ulterior intent is an additional aspect to the mens rea element of a crime that represents intention to create some additional effect beyond intent or recklessness to commit the crime.
It is not as common as basic intent or specific intent in criminal law, but the concept of a hidden or secondary intent can be relevant in determining a defendant's guilt or innocence, as well as the appropriate sentence or punishment.
In criminal law, the prosecution must prove that the defendant had the required mental state or intent to commit the crime. This can include basic intent or specific intent. Normally, the prosecution does not need to prove any ulterior intent beyond what is required for the specific crime.
Offences of ulterior intent are those that require proof of a second mens rea element. For example, under S9 of the Theft Act 1968, a person is guilty of burglary if he enters a building as a trespasser with intent to theft, causing grievous bodily harm, or causing criminal damage. The first intent is trespass as he enters the building, and the additional intent is theft as he steals something. The additional intent is the ulterior intent that must be proven before he can be convicted of burglary.
You can learn more about this topic and relevant case law with our Criminal Law notes.