The majestic St Edward's Crown - with which the Queen was crowned - is to play a symbolic role as the monarch marks the 60th anniversary of her coronation.
It will be brought to Westminster Abbey to rest on the High Altar as the Queen and the Royal Family gather in the church to commemorate the historic milestone.
It will be the first time the solid gold crown, which glitters with jewels, has left the Tower of London since the 1953 coronation 60 years ago.
The crown was set with tourmalines, white and yellow topazes, rubies, amethysts, sapphires, garnet, peridot, zircons, spinel, and aquamarines
The crown was made for the coronation of Charles II in 1661 and was a replacement for the medieval crown which was melted down on the orders of Oliver Cromwell in 1649.
The Ampulla, the gold, eagle shaped bottle from which the holy oil was poured for the anointing - the most sacred part of the coronation - will also be moved from the Tower to rest on the Altar.
During the service, a flask of aromatic oil - taken from the same batch made for the Queen's coronation - will be carried by representatives of the people of the UK through the Abbey to the Sacrarium, received by the Archbishop of Canterbury and placed by the Dean of Westminster on the High Altar.
In a foreword in the Order of Service, the Dean The Very Rev Dr John Hall said: "Our intention in this service of thanksgiving is to evoke and reflect the shape of the Coronation service itself."
He described the presence of the priceless, glittering St Edward's Crown in the Abbey as "a powerful symbol of the moment of Coronation".
The service has been divided into the Recognition, Anointing, Homage and Thanksgiving, reflecting parts of the original Coronation ceremony.
Just as in 1953, when the Queen enters the Abbey, her arrival will be marked by the choirs singing Psalm 122 - I was glad - set to Edward VII's coronation music by Sir Hubert Parry, as well as exclamations of Vivat Regina Elizabetha (Long Live Queen Elizabeth) by the Queen's Scholars of Westminster School.
The Recognition will centre on giving thanks for the Queen's "dutiful service".
For the Anointing, the oil will be processed through the Abbey from the Great West Door and placed on the Altar, but not used.
Ahead of the Anointing, Prime Minister David Cameron will read from the Old Testament Book of Kings, where the anointing of Solomon as King is described.
The Homage is reflected in a poem, written for the occasion by the Poet Laureate Carol Ann Duffy, and a prayer.
On show will be the restored Coronation Chair, which dates from around 1300 and was made on the orders of King Edward I.
In 1953, after the Holy Communion, also known as the Holy Eucharist meaning Thanksgiving, the choir sang a hymn of praise, Te Deum, set to music by William Walton.
Today's service will end with a hymn of thanksgiving and the Walton Te Deum.
At 2.30 pm the Westminster Abbey Company of Ringers will ring a Coronation 60th Anniversary celebratory peal of London Surprise Royal, made of 5060 changes.
In honour of the anniversary, the Abbey has renamed its Triforium Galleries as The Queen's Diamond Jubilee Galleries.
It has launched a major campaign to raise funds to enable proper access and to open a new museum in the Galleries, which stand 70ft high and were built in the 13th century by Henry III at the east end of the landmark.
The Queen was crowned by the Archbishop of Canterbury on June 2, 1953.
Weighing 2.23kg (nearly 5lb), the St Edward's Crown is the heaviest crown in the collection of Crown Jewels.
Dr Geoffrey Fisher raised it aloft and placed it on the monarch's head. ''By a glance she indicated it was steady,'' he later recalled.
It is set with tourmalines, white and yellow topazes, rubies, amethysts, sapphires, garnet, peridot, zircons, spinel, and aquamarines, step-cut and rose-cut and mounted in enamelled gold collets, and with a velvet cap with an ermine band.
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