The Prison Service said that at any one time within the constantly changing prison population of about 49,000 there are only about 35 known prisoners in England and Wales who are HIV positive. The national figure - including Scotland and Northern Ireland- is nearer 50.
The Prison Reform Trust, however, believes the true number could be ten times as great and is concerned that underestimates may make prison authorities complacent about the risks of HIV transmission. ``Shared needles by injecting drug users is probably the key element in the spread of infection in jails,'' it said.
A full survey of English and Welsh prisons is to begin ``shortly'' to discover the true prevalence of HIV, according to Ahilya Noone, who will carry out the research for the Government's Communicable Disease Surveillance Centre in London.
A pilot survey of prisoners was carried out in 1993. Just over half of a sample of 1,150 men eligible to take part actually did so. The results, published last week, showed that 0.3 per cent - two out of 584 - were HIV positive. But researchers were concerned that the large proportion of prisoners who refused to take part may have skewed the data.
Dr Noone said the results will be anonymous and not linked to any possible identifying details. Prisons in the London area will be first to take part in the survey. ``We plan to look at eight to 10 prisons in a year." Similar surveys of Scottish prisons have shown wide differences in the proportion of prisoners willing to take part in HIV surveillance programmes.
In 1993, only 43 per cent of 378 inmates in one prison agreed to be tested. A dozen prisoners - 7 per cent - proved to be positive. Last year, 92 per cent of the 1,073 inmates of Barlinnie Prison in Glasgow took part in a similar survey and only nine - about 1 per cent - were positive.
So far there are only two confirmed cases of prisoners becoming infected with HIV whilst in prison. Both occurred in a Scottish prison where needle sharing had taken place.
An Aids advisory committee has made a number of recommendations on how to limit the spread of HIV in prisons ``which are currently being considered'', according to the Prison Service.
Although educational videos and the issuing of protective clothing to prison staff have been introduced, the more controversial suggestions have not, the Prison Reform Trust said.
Proposals to issue syringe/needle exhanges and condoms have been made by a variety of agencies. They have not been accepted by the Prison Service, the Trust said.
``The argument against needle exchange is that it would condone illicit drug use, especially at a time when the Prison Service is planning a `war on drugs' in the prisons.''
The Prison Service, however, said there is evidence to suggest that many of those who have injected drugs before entering prison stop doing so once in prison.
``It is a question of balancing the risk of increasing injecting within a prison by providing needles against the risk faced by a small number of inmates who continue to inject.''