British Queen Consort Camilla will wear Queen Mary's Crown at her and King Charles III's coronation in early May, Buckingham Palace said on Tuesday.
The crown is more than a century old and was commissioned by Queen Mary for her 1911 coronation alongside King George V. It will be modified for the landmark ceremony on May 6.
This is the first time in recent history that an existing crown will be used for the coronation of a consort instead of a new commission being made, according to the palace.
It said the move was being made "in the interests of sustainability and efficiency".
The crown will be reset with several Cullinan diamonds -- cut from what was the largest diamond ever mined when it was discovered in South Africa in 1905 -- in tribute to the late Queen Elizabeth II.
The diamonds were part of her personal jewellery collection for many years and she often wore them as brooches.
Some of the diamonds were set into Queen Mary's Crown for the 1911 coronation and for King George VI's coronation in 1937, when it was worn as a regal circlet.
It has been removed from its usual display at the Tower of London for the modification, which will also include some other minor changes and additions by the crown jeweller.
Four of its eight detachable arches will be removed to create a new impression, different from that of 1911.
King Charles will wear the St Edward's Crown for the coronation service in Westminster Abbey. It has already been modified and returned to public display in the Tower of London.
- Concert ballot -
Preparations for the May 6 crowning, set to be attended by dignitaries from around the world and watched by billions, continue to gather pace.
The palace announced last month that the weekend-long celebrations will feature a star-studded concert, nationwide "big lunch" and volunteering initiative, as well as the traditional ceremony and royal processions.
Last week it unveiled a "coronation emblem", which will feature throughout.
The emblem pays tribute to Charle's love of the natural world, depicting the floral symbols of the UK's four nations -- the rose, thistle, daffodil and shamrock -- in the shape of St Edward's Crown.
Also last week, an online national ballot opened so 5,000 members of the public can be selected to receive a pair of free tickets for the concert in Windsor Castle's grounds on May 7.
After the ballot closes at the end of this month, the 10,000 tickets will be allocated based on the geographical spread of the UK population, and not on a first-come first-served basis.
Camilla, 75, has been forced to pull out of public events this week after testing positive for coronavirus, the palace said on Monday. It is the second time she has caught the disease.