Prison healthcare is still not good enough - four years after it was revamped to bring it up to NHS standards, a report claims.
The joint Care Quality Commission and Inspectorate of Prisons study said while there were signs of improvement, a number of areas of weakness remained.
In particular, it highlighted drug treatment as a major concern.
The Department of Health said good progress had been made since the survey was carried out in 2007.
The inspectors carried out reviews of 21 NHS trusts, responsible for more than a quarter of England's jails.
All prisons have some form of medical centre, often with their own pharmacies, nurses and doctors and with access to regional hospitals for patients who require more intensive care.
The standard of prison care has been in the spotlight since the mid 1990s when the then chief inspector of prisons, Sir David Ramsbotham, called for change.
In 2006, primary care trusts, which are in charge of local NHS services, took over responsibility for care from the prison service.
The report acknowledged improvements had been made since then.
Governance arrangements were more robust, investment had driven up overall standards and staff training was better, it said.
However, the regulators had concerns over continuity of care - the arrangements in place for transfers between centres and for release - which were said to be "inadequate and getting worse".
Only one trust had a specific policy in place.
Meanwhile, services for drug misusers were not up to standard, with only six of the 21 trusts adhering to best practice in areas such as the prescribing of drugs substitutes and counselling.
There was also a lack of evidence of good dentistry and health promotion practices, the report said.
Alex Baylis, from the CQC, said "significant improvements must be made".
"People in prison are often not registered with a doctor and have a high prevalence of chaotic lifestyles, which can involve - for example - substance misuse and mental health issues," she added.
Commenting on the report Juliet Lyon, director of the Prison Reform Trust, said: "Prison has become a capacious net into which those let down by other public services fall.
"Mental illness, addiction, sickness and disability are far more prevalent in custody than in the rest of the community.
"Yet as this report highlights, NHS healthcare is often ill-equipped to meet the standards of care it is legally required to provide."
She added that the government's emphasis on moving addicts and the mentally ill away from the justice system "must go hand in hand" with suggestions contained in the report.
Care Services Minister Paul Burstow said: "Offenders are more likely to have poor health so the system designed to support them must be robust.
"While there is a long way to go, good progress has been made since this survey in 2007. The Department [of Health] is working closely with the NHS and partners to make sure offenders have access to the care they need when they need it."