The report by the All-Party Parliamentary Drugs Misuse Group says that short term and remand prisoners - those most likely to suffer drug addiction - receive little or no help to come off drugs.

This is because they are not subject to sentence planning arrangements.

The report says drug use is made worse by prison overcrowding and cutbacks in education and training programmes.

"This can lead to drug use to cope with the numbing effects of boredom and inactivity," it says.

Drug treatment programmes, supported by the government in its recent White Paper on drugs, are often patchy.

"The mismatch between national strategy and local implementation is distressingly large in some prisons. This is equally true in terms of the quality of health care and drug treatment provision," says the report.

Hepatitis

The members of the group are particularly worried about health threats, such as Hepatitis C. Drug users run a high risk of infection from the liver disease because of shared needles.

Prisoners are taking heroin to avoid detection

They recommend that the Department of Health and the Prison Service work together on a national strategy to address the problem.

The report says there is evidence to show that use of cannabis in prison leads directly to hard drug misuse.

It has heard evidence from drug users who say many prisoners switch from cannabis to harder drugs, such as heroin, in order to avoid detection in tests.

And some binge at the weekends when no testing takes place.

Training

The group recommends that prison staff, judges and magistrates be given extra training about drug misuse.

It says this would encourage the courts to stop "in appropriately" sending people to prison for minor drugs offences.