It is the second death recorded at the Pembury Road station in the last four years.
The woman who died has not been named and was arrested for an undisclosed offence by officers from Kent Police at around 8pm on Friday July 23.
At about 1pm the next day, she appeared to be having trouble breathing and was found unresponsive in her cell.
Paramedics were called and despite efforts to save her she was pronounced dead at the scene.
The coroner was informed, but a post-mortem examination that took place on Sunday July 25 was inconclusive as to her cause of death, pending further tests.
A date has yet to be fixed for the inquest.
The force referred itself to the IOPC following the death and investigators were sent to Tonbridge Police Station, where they have preliminarily interviewed officers and visited the custody suite.
IOPC Regional Director Graham Beesley said: “My thoughts are with the woman’s next of kin and all those affected by her death.
“We have made contact with her next of kin and explained our role to them.
“It is important we fulfil our role to carry out an independent investigation into all the circumstances surrounding her contact with the police.
“We will examine all the actions and decisions of officers and staff involved in her detention including viewing custody records and CCTV as well as the welfare checks that were made on her during that period.
”Our investigation is in its very early stages.”
In 2017, the Independent Police Complaints Commission [IPCC] – the forerunner to the IOPC – investigated Kent Police following the death of a 29 year old from Maidstone at Tonbridge Police Station.
Carl Maynard died while he was having his fingerprints taken at the station on October 13 of that year.
An inquest found he had been killed by acute toxicity after he had swallowed a bag of cocaine to avoid the drug being detected by officers.
The IPCC found no evidence of any wrongdoing by any officer following Mr Maynard’s death, although a coroner did criticise the force for ‘failing to take Carl directly to hospital’, which he said was a ‘missed opportunity’ to save the man’s life.
The force was also placed under investigation by the IOPC last year following the death of Matthew Mackell in Dunorlan Park.
The 17 year old had ended his own life after calling Kent Police and saying he was ‘going to kill myself’.
Despite the plea for help, no officers were despatched to the park. The IOPC found no evidence of any wrongdoing or misconduct by any officer over the incident, but did recommend two people at the force conduct extra training.
A coroner in that case also ruled there had been a ‘missed opportunity’ by the force after it failed to send an officer to the park.
Following the latest incident, a spokesman from Kent Police said: “Kent Police made a mandatory referral to the Independent Office for Police Conduct (IOPC) following the death of a woman in custody on Saturday, July 24.
“South East Coast Ambulance Service was called to Tonbridge Police Station shortly before 1pm after the woman, who was in her 40s, was found unresponsive in her cell during a routine welfare check.
“She was pronounced dead at the scene.
“Investigators from the IOPC attended the police station and have commenced an independent investigation into the circumstances.”
