One in four prisons in England and Wales are holding transgender inmates, an official report has revealed.
Data collected in March and April showed that 33 of the 123 public and private prisons reported they had one or more transgender prisoners.
There were 19 establishments with one transgender inmate, ten prisons with between two and four, and four facilities holding at least five.
A paper published by the Ministry of Justice (MoJ) said there were 70 inmates currently living in, or presenting in, a gender different to their sex assigned at birth and who have had a "case conference".
It did not include information on the number of prisoners who have already transitioned and have a full gender recognition certificate.
Of the 70 inmates, 52 reported their gender as male and 14 as female, while it was not stated by four.
The report said: "The gender is self-reported on reception to prison and based on information recorded on central administrative databases.
"It is not possible to determine if this is the legal gender or whether or not the gender has changed."
There will be some transgender prisoners who have not had a case conference and some who are not known to prison staff, according to the study, so there may be some under-counting.
Ministers launched a review of the care of transgender prisoners last year after the deaths of two inmates being held at all-male prisons.
The review, published by the MoJ on Wednesday, said: "It is apparent that the treatment of transgender people in courts, probation and prison services has not kept pace with the development of a more general understanding of the issues surrounding gender in society."
It said allowing transgender offenders to experience the criminal justice system in the gender in which they identify "will, in the great majority of cases, represent the most humane and safest way to act".
It added: "We believe it will also assist successful rehabilitation.
"In the minority of cases where that is not possible, the reasons for departing from this starting presumption must be clear, explicit and made known to the person they affect, especially when it involves assigning someone to a male or female prison."