If you happen to possess any cartoon images on your hard drive – or on your bookshelf – that just might depict children involved in or present at a sexual act, then you should probably have deleted them already.

Today – April 6 2010 – is the day on which various sections of the Coroners and Justice Act 2009 come into effect, including those (sections 62 to 68) that specifically criminalise possession of "a prohibited image of a child". The purpose of this offence is to "close a loophole" and to target certain non-photographic images of children, possession of which is not covered by existing legislation.

Henceforth, you will be committing an offence if you possess non-real, non-photographic images that are pornographic, "grossly offensive, disgusting or otherwise of an obscene character" and focus on a child’s genitals or anal region, or portray a range of sexual acts "with or in the presence of a child".

Hitherto, the state of play in the UK - courtesy of the Protection of Children Act 1978 (pdf) - was that possession, creation or distribution of an indecent image of a child was a criminal offence. Legislation and precedent extended the law to cover "pseudo-photographs of children": that is, images that were created by means of the manipulation of real, non-indecent images of children and adults, to create an end result that a jury deemed indecent.

Finally, the Sexual Offences Act 2003 raised the age limit for such imagery to 18 – thereby creating the novel legal concept that an act that might legally be viewed in real life became criminal as soon as it was committed to film (or pixels).

The rationale behind the original law was that every indecent image of a child was also an image documenting the abuse of a child. The extension of such a law to "pseudo-photographs" strained at that principle, although in cases where a real identifiable individual was featured, the logic – of protecting that individual from the traumatic effects of discovering or being aware of the existence of such imagery – remained intact.

Debate on the new law made clear that two additional principles were being called into being. The first – for which evidence is contradictory – is that there is a natural progression in the expression of paedophilia, with individuals moving from fantasy to acting out of the fantasy. In such cases, images are considered to encourage such a progression.

The second motivation is more controversial and appears to stem from a knee-jerk "if paedophiles enjoy it, it must be bad". Thus, in the original consultation, one police force declared its upset at being unable to do anything in respect of an individual who possessed nothing other than indecent cartoon imagery, and being required by law to return the material it had confiscated.

Otherwise, the thinking behind this legislation appears typified by Labour MP George Howarth who, during the committee stage of the Bill, famously observed, of a drawing scrawled on a piece of paper: "If somebody is in the process of arousing themselves sexually by that process, it must be part of something. In a lot of cases, it will be part of something that will lead on to something else."

Critics have objected to this legislation on two grounds. The first, that if there is now a criminal penalty in respect of possessing cartoon imagery, this may have the unintended consequence of lessening barriers to the possession of real imagery. There is also some concern that whether a cartoon falls into this category or not will depend on whether a jury decides the age of a cartoon image was over or under 18.

Also, within the small print of the law, is the issue of when a sexual act may be deemed to take place "in the presence of a child". Same frame? Same room? Same cartoon, but different page?

The Ministry of Justice has declined to answer - although a court’s interpretation of that wording may at some future time determine whether an individual goes to prison, or walks free. ®