Republic has vowed to launch a protest during the Trooping the Colour celebrations on June 15.
The Anti-monarchy group announced they will pitch up on The Mall near Buckingham Palace with banners in protest of the King and Royal Family.
In a statement posted to X, the group said: “On Saturday, June 15, Charles will be having his official birthday celebration parade, at Trooping the Colour.
“This is another costly celebration of a corrupt and dishonest institution, at a time when people are crying out for change.
“So join Republic supporters down on the Mall to protest against the monarchy.
“Charles will be part of the procession, along with other royals.
“We'll be there too, to say loud and clear that it's time to abolish the monarchy.
“Republic protests are a lot of fun and hugely important to the growing movement for a democratic future. Pledge to be there, and we'll see you on Saturday June 15.”
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The announcement comes after the royal palaces confirmed King Charles will be in attendance at this year's Trooping the Colour event, but Princess Kate will be absent as she continues her recovery.
Some 2,000 soldiers and around 240 horses from the Household Division will march in the King's Birthday Parade.
Detailing the news from the Palace, GB News Royal Correspondent Cameron Walker says Kate's absence means she needs “more time and space to recover” as she continues her preventative chemotherapy treatment.
Cameron told GB News: “The Princess of Wales, she is Colonel of the Irish Guards. It is the Irish Guards this year who are trooping their colour in front of the Sovereign.
“And usually, tradition would be that she would take part in the Colonel's review the week before, on June 8. But due to her ongoing cancer recovery, that is not happening.”
General James Bucknell is reported to be taking part in the review on Kate’s behalf.
Cameron added that the monarch will be making his appearance but on an “adapted scale”.
Cameron explained: “They have confirmed that His Majesty the King will be taking part in the King's birthday parade, Trooping the Colour, on June 15. But adaptions have been made because he is still undergoing cancer treatment.
“So instead of riding on horseback, the King is going to be riding in a carriage from Buckingham Palace with Her Majesty the Queen down the Mall to Horse Guards Parade, and will inspect the troops from a carriage rather than on horseback.”
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