The importance of access: Andrew Ridley interviewed on BBC Radio 5 Live

The trust is concerned that no national guidelines exist for the treatment of HIV-positive and Aids patients in jail, and that strict prison regimes often prevent effective treatment.

The warning follows a survey by the National Aids and Prisons Forum which found that some infected prisoners were receiving no treatment at all, while others were being given their medication at haphazard intervals.

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The Terence Higgins Trust is an HIV and Aids charity

Andrew Ridley, director of operations of the Terence Higgins Trust, said it was extremely important that patients receiving combination therapy should continue to have access to it when in prison.

Otherwise, he warned, the type of HIV a patient was suffering from can "learn to become drug-resistant to the drugs that you've been taking".

"The worry from a public health perspective is that when they are released back into the community, they may take back with them a drug-resistant strain of HIV which they will then pass on to other members of the community," he told BBC Radio 5 Live.

Complicated combinations

Combination therapy can be very complicated. Patients often need to take a number of different drugs at different times of the day.

Mr Ridley said that though this could be difficult for prisons to organise, such treatment must still be available to those in jail who need it.