Police are investigating the death of a baby in Britain’s largest female prison after an inmate gave birth alone in her cell at night.

The Guardian understands that the woman, who had been at an advanced stage of pregnancy, gave birth alone in her cell in the early hours of Friday last week. A source with knowledge of the events said that when prison staff visited the woman’s cell on Friday morning the baby was unresponsive.

Vicky Robinson, the director at HMP Bronzefield, confirmed that a baby died at the prison last Friday and said it was supporting the mother.

South East Coast Ambulance Service confirmed it had received a call from the prison at 8.30am last Friday and that one ambulance attended the prison. Police were called shortly afterwards.

Surrey police said in a statement: “The death is currently being treated as unexplained and an investigation is continuing to establish the full circumstances of what happened.”

The case raises serious questions about how the woman came to be unsupervised and without medical support during her labour and birth, and about the conditions at the privately run prison.

A spokesman for the Prisons & Probation Ombudsman, an independent body which investigates deaths in prisons and detention centres, confirmed that they are not investigating the baby’s death and said it was not part of their remit.

Robinson said: “We are supporting the mother through this distressing time and our thoughts are with her, her family and our staff involved. We are undertaking a full review and working with all relevant authorities during their investigations. It would not be appropriate to comment any further.”

Bronzefield is Europe’s largest female prison, holding up to 557 inmates, and is operated by Sodexo Justice Services. Concerns have been raised previously about the care of prisoners.

In the case of Natasha Chin, who died at HMP Bronzefield in 2016, an inquest jury found that neglect and systemic failings at the prison had contributed to her death. Four women have died at Bronzefield since July 2016.

There is a mother and baby unit at the prison and women whose babies are born in prison, or who already have a baby when they enter prison, can keep the infants with them until they are 18 months old.

Every year, about 600 pregnant women are held in prisons in England and Wales, and about 100 babies are born there.

According to the Prison Reform Trust, women made up 5% of the prison population in 2018 with 7,745 women incarcerated. The majority of them, 82%, were sentenced for non-violent offences.

Naomi Delap, the director of the charity Birth Companions, which supports pregnant women and new mothers in prison, said: “We are deeply saddened by the report of this incident in prison custody. Our thoughts are with the mother and all those affected by this tragic event. While the circumstances are still unclear and subject to ongoing investigation, this is a powerful reminder of the need for us to all ensure that pregnant and postnatal women have access to high-quality physical and mental healthcare in custody.”

Jane Ryan, a solicitor at Bhatt Murphy Solicitors who specialises in cases of women in prison, said: “There are serious concerns raised about risk issues arising from lack of access to midwives when labour commences, access to pregnancy records and full maternity care. It is an indictment of our prison system that a woman was giving birth in a prison cell. There must be a full independent investigation, and possibly an inquest, into these horrific circumstances.”

Dr Kate Paradine, the chief executive of the charity Women in Prison, said: “All mothers should have the right to give birth in dignity. But time and time again, we hear of situations where this isn’t happening, with mothers regularly being denied access to the vital health and maternity care necessary to give birth safely. This has devastating consequences not only for the baby and mother, but also for the rest of the family they are separated from.”

Deborah Coles, director of the charity INQUEST called for an independent inquiry into the baby’s death. “This is horrifying and exposes the utter inhumanity of a prison system where a woman can give birth alone in a cell and the baby dies.. The death of a baby behind bars warrants an independent inquiry. The fact that the PPO is not investigating makes this even more essential.”