The service, which will begin at 11.45am, will be led by the Reverend Prebendary Jonathan Osborne MBE, Senior Chaplain, Metropolitan Police Service, and Maori speaker the Venerable Jo Kelly-Moore, Archdeacon of Canterbury.
Speakers at the service will represent family, friends and police colleagues, including a tribute by Metropolitan Police Commissioner Cressida Dick.
Due to Covid restrictions, the Met Police is unable to provide a force 'service' funeral, but a full 'service' memorial will be planned when restrictions are lifted.
Following the service there will be a private cremation service.
Sgt Ratana, who was particularly well-known among Sussex's rugby community as a coach at East Grinstead Rugby Club, was killed in Croydon Custody Centre on September 25 after being shot by a suspect who had been detained in handcuffs.