Britain said on Wednesday that it will provide free sanitary products to all female and transgender male prisoners to help ensure menstruating detainees are "treated with dignity".

The Home Office (interior ministry) said it would change the law to make it mandatory for police to ask every female detainee at the earliest opportunity if they need free pads or tampons.

All detainees will also be able to speak in private to a police staffer of the same sex about their health, welfare and hygiene needs, the government said.

The Home Office told the Thomson Reuters Foundation that transgender male detainees will also be able to request period products and speak privately to a staff member of a sex of their choosing.

"I have been clear that everyone who enters custody should be treated with dignity and have their personal needs met," Nick Hurd, minister for policing, said in a statement.

The Independent Custody Visiting Association (ICVA), which works to improve custody conditions, said some women had been stripped of all clothing, including underwear, and placed in paper suits in police cells without sanitary products.

Britain had about 3,760 female detainees in 2018 - who accounted for less than five percent of the total prison population - ministry of justice figures showed.

"These changes ensure that the needs of female detainees are addressed, that detainees have basic privacy to use a toilet and access to menstrual products and that dignity is promoted within the police custody environment," said ICVA's head Katie Kempen.

"No detainee should be left to bleed for want of a difficult conversation or a cheap tampon. These changes should ensure that never happens," she said in a statement.

Gabby Edlin, founder of charity Bloody Good Period which supplies sanitary products to vulnerable women such as asylum seekers, said period products should not be viewed as a luxury.

"It is a human right to experience life with dignity regardless of where you are, if you've committed a crime," she said in a phone interview.

"It's just time that we stop considering this as a luxury. It's just something that half the population go through. It doesn't even depend on a gender."

Britain in March launched a global "period poverty" fund and taskforce to help all women and girls access sanitary products by 2050 and to tackle the stigma around menstruation.