Prince Charles will attend the ceremony in the centre of Bridgetown, the biggest town in Barbados, as the royal standard will be lowered for the last time and the presidential standard hoisted instead.
It’s the first time since 1625 that a British Monarch has not overseen this Caribbean island.
It gained independence from Britain in 1966 but tonight - on the 55th anniversary of independence - Barbados will transition from a Commonwealth Realm to a republic.
Prince Charles was invited to be here for the official ceremony by the country’s Prime Minister, Mia Mottley, as she was keen this moment was not seen as a snub to the 95-year-old British Monarch.
“I am very pleased that my son is with you today”, the Queen said in her first message to the new republic.
The Queen called it a “significant occasion” and spoke of when she first came here just ahead of Independence Day in 1966.
She said: “Since then, the people of Barbados have held a special place in my heart; it is a country rightfully proud of its vibrant culture, its sporting prowess, and its natural beauty, that attracts visitors from all over the world”.
Overnight, the number of realms - countries where she remains the head of state - reduced from 16 to 15, but Barbados will remain in the Commonwealth - the organisation of 54 mostly former British colonies.
The Queen, who is Head of the Commonwealth, wrote: “It is also a source of great satisfaction that Barbados remains an active participant within the Commonwealth, and I look forward to the continuation of the friendship between our two countries and peoples”.
She called Independence Day 2021 in Barbados a “momentous day” and it breaks a link with the British Crown which goes back nearly 400 years.
English settlers claimed this island in the Eastern Caribbean for King James I in 1625.
But the period of slavery from the mid-1600s to 1834 remains a stain on the British history on this island - and many others.
It is an issue that Prince Charles will address tonight in his speech to the crowds here.
The future King - and future Head of the Commonwealth - will reference the dark days of slavery and the British High Commissioner in Barbados expressed his deep sorrow for that period.
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Last year, Barbados approved the removal of a statue of Admiral Lord Nelson from National Heroes Square in Bridgetown - the scene of tonight’s official republic ceremony.
The bronze statue of Nelson was erected by British rulers in 1813 following victory at the Battle of Trafalgar but because of his support of slavery - it was deemed inappropriate in 2020 for it to remain in place.
It’s been moved to a museum and an empty plinth now stands where his statue used to be.
As Barbados begins a new chapter in its history - the debate about its colonial past and about the potential for reparations will continue.
The new head of state of Barbados is the former Governor General Dame Sandra Mason who will be sworn in as the first president.