Experts believe the legality of the Home Office practice could be tested in the European courts. They say it could amount to a breach of Article 8 of the Convention on Human Rights which guarantees the right to privacy.
The issue is being raised with Peter Lloyd, the Home Office minister, by the National Association of Probation Officers (NAPO), which is also concerned about who has access to the taped conversations and whether they can be passed to police.
Sophisticated recording equipment is being installed alongside card-phones, currently being introduced in the country's 128 prisons as part of penal reform suggested by the Woolf report into the Strangeways riots. The report recommended strengthening links between inmates, their families and the community at large by making telephones available and by ending censorship of mail.
But while staff are no longer opening prisoners' letters, they are monitoring the telephone calls of most inmates - including unconvicted remand prisoners.
Recording and listening in may take place simultaneously, or tapes may be played back later. Staff are told that if they realise the call is between prisoner and lawyer, the recording should be switched off. Similarly if they detect a distress call to organisations such as the Samaritans or an Aids helpline, they should observe confidentiality.
But penal reform groups argue this leaves too much trust and discretion to staff. They suggest that any monitoring should be restricted to high-risk prisoners. If there is desire for a particular inmate's calls to be scrutinised, an application for a telephone tap should be made to the Home Secretary in the same way as for a domestic telephone.
Andrew Puddephatt, general secretary of Liberty, said: 'A remand prisoner is not convicted of any crime and any prisoner still maintains basic human rights - one of which is private communication with their family or loved ones and with their legal representative. The automatic taping of all conversations and monitoring at the discretion of the governor or other prison officers is a violation of that right.'
The Home Office yesterday defended the monitoring, saying it was designed to stop abuses - such as prisoners making distressing or threatening calls to victims or witnesses.
A spokeswoman said that all the card-phones in the 60 jails where they are already installed were being used without complaint. Signs warn that calls are recorded. Other telephones were available for those wishing to ensure client-lawyer privacy.
Harry Fletcher, assistant general secretary of NAPO, said: 'Telephone installation was a long overdue innovation. But if there is abuse, then action should be taken against the individual concerned, rather than penalising the entire prison population.'