The 45-year-old was arrested at around 3.30pm on March 31 and was taken to the Romford custody suite.
He was charged and remanded in custody to appear in court on April 2.
At around 3.30am on April 2, custody staff found the man unresponsive during a cell check, the Independent Office for Police Conduct (IOPC) said.
CPR was given by police staff, and an ambulance was requested, however, he was pronounced dead at around 4am.
A post-mortem examination was completed on April 7, and the watchdog is now waiting for the results of further testing.
The Coroner has been informed and an inquest will be opened in due course.
IOPC director Amanda Rowe said: “Our thoughts are with the man’s family and friends, and all those affected by his death.
“We have now met with his family to explain our role and have advised them of the next steps.
“It’s important that an independent investigation is carried out to fully establish the circumstances when someone has died in police custody.
“We will ensure that the man’s family and the force are kept informed as our investigation progresses.”
IOPC investigators are reviewing CCTV footage from the custody suite and body worn footage from the arresting officers.
It is also reviewing custody logs and will be obtaining statements from the relevant custody staff involved.
