Theresa May has confirmed the independent probe will examine the lead-up to and aftermath of fatalities and serious incidents, with a focus on the support given to bereaved families.
It will also include an investigation of issues including the use of restraints, suicides that take place within 48 hours of being detained and access to mental health facilities.
The Home Secretary said every death in custody has the potential to "underline dramatically the relationship between the public and the police".
It comes as figures show the number of people dying in, or after being released from, police custody reached 17 last year - the biggest number in England and Wales in five years.
On Premier's News Hour the Executive Director of the Christian Police Association Don Axcell said the figure was less than half of what it was when records started in 2006.
But he added it didn't "diminish the seriousness of the figure".
People "may die as a result of vomiting whilst unconscious through alcohol despite the number of checks made on them," he said.
He added that police officers were having to make on the spot decisions and react within seconds.
"Once he decides to arrest then the person he is arresting decides to resist and the situation can escalate in no time at all.
"Who knows what can happen when you're having to literally fight with someone in the street, often on your own."
Premier's Marcus Jones speaking to Don Axcell on the News Hour: