Almost a fifth of women who died in prison during the past two years had been born men, according to official figures.

The Prison and Probation Ombudsman recorded 33 female deaths in custody between 2016 and 2018. Of those, six were men who ‘self-identified’ as women.

Critics last night warned that the inclusion of transgender women risked skewing the figures and creating confusion.

Richard Garside, director of the Centre for Crime and Justice Studies, said: ‘I hope the ombudsman reviews its approach and starts classifying accurately these tragic cases. The causes and contexts of male and female deaths in prison are not always the same.

Transgender Vikki Thompson, 21, (right, pictured with boyfriend Robert Steele) was found dead at men's jail HMP Leeds in 2015

‘If the PPO wants to end the confusion, increase understanding and prevent avoidable deaths in the future, it should provide a separate breakdown of data on transgender prisoners.

‘This would also help us understand the unique problems that transgender inmates face.’

Nicola Williams, of campaign group Fair Play For Women, said: ‘Every death in custody is a tragedy and such poor data makes it impossible for us to tell what’s going on. Political correctness must not lead to women and transgender inmates having distinct needs ignored.’

The PPO report records 23 deaths of women in custody in 2016-17, of which four were men who identified as female. Ten female inmates died the following year, including two transgender women who were born male.

Thirteen of the deaths over the two-year period are described as ‘self-inflicted’, but the report does not specify which of these involved transgender prisoners.

The Prison Service updated its policy in November 2016 following a spate of deaths of inmates who had transitioned from male to female, now ruling that transgender prisoners ‘must be allowed to express their gender identity’.

Deaths of prisoners like Thompson is leading to skewed figures of the amount of female deaths in jail, critics warned

Transgender deaths in jail include Joanne Latham, 38, who was found hanged at Woodhill prison in Milton Keynes, and Vikki Thompson, 21, who died at Armley jail in Leeds. Both cases happened in November 2015. Nicola Cope died at Foston Hall women’s prison in Derbyshire in November 2016, and Jenny Swift, 49, was found dead at the all-male Doncaster jail a month later.

Whitehall sources last night confirmed that the PPO records a prisoner’s gender based on how they identify themselves – a decision that comes amid controversy over male inmates being allowed to identify as women without any formal medical diagnosis.

There was outcry last summer when it emerged that transgender prisoner Karen White – who was convicted of two rapes on women as Stephen Wood – had sexually assaulted two prisoners after being transferred to a women’s jail despite not undergoing any sex-change surgery.

The Ministry of Justice says there are now 139 transgender prisoners in England and Wales, double the number in 2016.

Last year, it was revealed that almost half of transgender inmates had been convicted of sexual offences.

The Prison Service said: ‘We cannot comment until the PPO has concluded its investigations.’