Drugs offenders, drugs related offending and the prison population
The ballooning size of the prison population has been increasingly prominent in the media and in political debate. The role of drug enforcement is, however, rarely acknowledged. A significant proportion of inmates are seeing out sentences for drug offences (under the Misuse of Drugs Act and related drug legislation) and an even greater proportion for a range of secondary drug related offences (more accurately prohibition related offences). Combined it is reasonable to suggest that drug law enforcement is directly responsible for over half of the prison population, although poor data and research means it is impossible to come to precise figure.
Crime, illegal drug markets and problematic use of illegal drugs are intertwined through a series of complex connections, but primarily because:
The influence of drug policy and prohibition on the prisons crisis is clear, and it is worth noting that the UK, (in contrast to the line promoted by many commentators and opposition politicians) has amongst the harshest and most punitive drug enforcement in Europe, amongst the highest per capita prison population in Europe  and consistently amongst the highest level of drug use and drug deaths in Europe .
Prison population data, unlike many other statistics in the drugs debate, is both reliable and up to date.
There are currently around 80,000 people in prison, the precise figure fluctuating on a daily basis. The total population is updated and published on a weekly basis by the home office. 
Rising prison population
Numbers in prison for drug offences have risen sharply over the last decade, faster than any other identified group of offenders. It is important to note that the biggest increase is for drug offences – of 5825 inmates (up around 250% for men and 300% for women). 
The Home Office, taking into account current trends, has estimated that the prison population will rise to 106550 by 2013. 
There are a higher proportion of black inmates on drug offence charges (28%) compared to white (13%). This is despite the black community having a per-capita level of drug use lower than whites . (There is some discussion of the overrepresentation of black drug offenders in the prisons here: ‘Race And The `Drug Problem' More Than Just An Enforcement Issue' Philip Guy).
The most recent data is that there are 10613 people in prison for specific drug offences. 
Number of Drug-Related Offenders
Trying to determine Drug-related crime statistics are much more problematic. This is because defining what crimes are ‘drug related' is difficult, and also the criminal justice system does not systematically collect data on this. Courts are concerned with the actual crime committed i.e. theft, fraud, violence, and not the motivation of the crime (e.g. need to pay for drugs, intoxication). However, some research has been done on drug-related crime, with estimates usually based on drug testing of arrestees, and/or information volunteered in interview by arrestees.
The NEW-ADAM programme (New English and Welsh Arrestee Drug Abuse Monitoring) is the most thorough undertaken by the Home Office and provides the best large-scale data on this question. Interpretation of this data differs however and there are clearly some methodological weaknesses in the survey.
The first NEW-ADAM analysis (1999) from the Home Office  first report found that:
Based at least in part on the NEW-ADAM research, the (2003) report from the Number 10 Strategy Unit  claimed that over half of all property crimes were drug motivated:
The results of the NEW-ADAM Programme 1999-2002  suggested a plateau of around 65% of arrestees testing positive for drugs. Of which 47% were cannabis users, 28% opiate, 5% methadone, 23% cocaine, 6% amphetamine, 18% benzodiazepines.
It is difficult to come to firm conclusions from any of this data. 'Drug related crime' is a nebulous concept at the best of times, and the data that has been collected is not methodologically strong and open to interpretation. We can really only make fairly broad statements that need to be caveated as inference/generalisations from a relatively poor research base. (The parliamentary search engine and www.theyworkforyou.com can assist you when searching for comments on drugs/prisons and related subjects made in various parliamentary fora).
Mr. Jeremy Browne : To ask the Secretary of State for the Home Department how many incidents of robbery reported to the police in 2006 were classified as drug-related crime. [PQ No. 117790]
Mr. Coaker : Data on offences of robbery recorded by the police are available from the recorded crime statistics. However, it is not possible to determine those that are drug-related as no information is collected on the circumstances surrounding the offences.
Drug Users (Illegal Income)
David Davis: To ask the Secretary of State for the Home Department what estimate he has made of the average annual illegal income of arrested (a) heroin and (b) crack users. [PQ No. 47717]
Mr. Charles Clarke: We do not have the information requested.
Drug Addiction (Crime)
Mike Penning: To ask the Secretary of State for the Home Department what assessment his Department has made of the proportion of crime undertaken as a direct consequence of drug addiction; and if he will make a statement. 
Paul Goggins: Crime statistics used for monitoring overall crime trends, such as recorded crime and the British Crime Survey, do not contain information about the drug habits of individual offenders or their motivation for offending. It is therefore not possible to provide firm estimates of the total amount of crime undertaken as a direct consequence of addiction.