Oscar-nominated Cumberbatch, who is to star as Richard III in an upcoming TV adaptation, will read a verse penned for the service by Britain's poet laureate, Carol Ann Duffy.
The 14-line verse, titled simply "Richard," will remain a closely-guarded secret until the service of reinterment at Leicester Cathedral on Thursday.
All the organizers will say is that it is a "meditation on the impact of [Richard's] finding and on the legacy of his story."
They also have revealed that it contains the phrase "grant me the carving of my name," recognizing that Richard III's grave was lost for centuries, with no tombstone to mark its location.
In a statement, Duffy said it was "a privilege to be involved, in a small way" in the farewell to Richard, the last English King to die in battle, 530 years ago.
Cumberbatch's reading will form part of a service of reinterment for the medieval monarch, which will be led by the Archbishop of Canterbury, Justin Welby.
Key figures in the rediscovery and identification of the royal remains, including historians, archaeologists, scientists and relatives are expected to attend, along with members of the public who won a ballot for tickets.
Queen Elizabeth II has written a message to be included in the order of service for the event, at which she will be represented by the Countess of Wessex.
On Sunday, tens of thousands of people lined the streets of Leicester
and its surrounding villages as Richard III's coffin was carried out to the site of the Battle of Bosworth, where he was killed in 1485, and back in a procession.
Since then, people have queued around the block to see it lying "in repose" in the cathedral where it will be reinterred in a tomb made of pale Swaledale fossil limestone from Yorkshire, set on a plinth of black Kilkenny marble.
Richard III's remains were found in August 2012 by archaeologists searching for the remains of the Greyfriars Monastery, which had been covered over by a council car park.
The skeleton has been studied at the University of Leicester ever since, allowing scientists to discover a host of details about Richard III's life -- and death
-- by analyzing the bones.
The university's genealogy expert, Kevin Shurer, has discovered that Cumberbatch is related to Richard III.
"Benedict is Richard III's second cousin, 16 removed," Shurer explained. "He is linked in several ways, but ... the shortest is via Richard's mother, Cecily Neville's grandmother, Joan Beaufort. He also has more indirect links to both Queen Elizabeth II and Lady Jane Grey through other ancestors in his [family] tree.
"It is great that Benedict has the opportunity to take part in the ceremony ... having him there will add another dimension to what has already been a momentous week."
Shurer also helped confirm the link between the last Plantagenet King and living relatives Michael Ibsen and Wendy Duldig, whose DNA proved the identity of Richard III's remains
Ibsen, a Canadian-born cabinetmaker who now lives in London, built the English oak and yew coffin
in which Richard III will be buried.