Thousands of officers from across the country are expected to line the streets as his coffin is taken from the Palace of Westminster at around 1.30pm, where it has lain in rest overnight, to Southwark Cathedral for the funeral at 2pm.
A private cremation will follow the service, where several officers are expected to take part as ushers.
The Queen gave permission for PC Palmer’s body to rest in Westminster’s Chapel of St Mary Undercroft, an honour normally reserved for heads of state.
Scotland Yard said that full service funerals are normally only held when a police officer or member of staff dies while they are carrying out their duty.
Chief Constable Sara Thornton, head of the National Police Chiefs’ Council, said the scale of the funeral will be unprecedented as officers hold a two-minutes’ silence at 2pm.
She told the BBC’s Victoria Derbyshire Show: “I don’t think we will have ever seen a police funeral of this size.
“Officers from all over the country are coming to London to join their Metropolitan Police colleagues to line the route.
“But at 2pm outside police stations across the country, officers and staff will be observing a two-minutes’ silence.
“We all want to pay honour to the ultimate sacrifice that Keith made.”
PC Palmer’s name was added to the roll of honour and remembrance at a ceremony at the National Police Memorial on The Mall, central London, complete with a guard of honour, hours before his funeral.