When King Charles III is crowned in a lavish ceremony next week, Britain's main anti-monarchist movement will gather along the procession route next to a statue of Charles I, who was beheaded in 1649, leading to a short-lived republic.
Supporters of Republic, a group founded in 1983 that campaigns for an elected head of state, are planning their biggest ever protest. They believe Charles' accession to the throne presents their best chance of ending the monarchy, which traces its history back more than 1,000 years.
Graham Smith, its head, sees the grand coronation at London's Westminster Abbey as the perfect opportunity to expose what he regards as an anachronistic institution with no place in a 21st century democracy, particularly at a time when people are facing the worst cost of living crisis in decades.
Protestors in gold
Smith said he expected over 1,000 people dressed in yellow shirts to join the protest on May 6, where they will hold up placards, and give speeches.
When the newly crowned king passes in his gold coach, through streets lined with tens of thousands of well-wishers, they plan to boo loudly and chant "not my king."
Polls show Charles is less popular than his mother Queen Elizabeth, the world's second-longest reigning monarch, whose death last year marked the end of an era in Britain.
According to YouGov, in 2012 73% of the public said the monarchy was good for Britain, but that figure has dropped now to 53%.
Smith said the public's respect for the queen meant she had been an "obstacle" to the republican cause.
"I think the monarchy is in a lot of trouble because they have lost their star player, support is clearly going down, interest is going down and that is a big problem for them," Smith told Reuters.
"Charles has not inherited the deference, respect and sycophancy that was enjoyed by the queen, so people are far more willing to challenge him."
The British monarchy traces its history back to William the Conqueror who invaded England in 1066, though royals ruled the patchwork of kingdoms which stretched across what became England, Scotland and Wales for centuries before that.
While the monarchy has gradually ceded power to parliament over the centuries, the king or queen still plays a significant, if almost entirely symbolic, role in British life such as the appointment of prime ministers and the judicial system.
Apart from the 11-year republic from 1649 following the end of the English civil war and the execution of Charles I, support for ending the British monarchy has rarely attracted significant popular support.
Anti-monarchy protests are relatively small, and polls show the majority of Britons still want a royal family.
But support for the monarchy is slipping and surveys indicate the young are less interested in the institution than older generations. YouGov this month also found only 9% of the population said they cared a great deal about the coronation.
Tony Travers, a politics professor at the London School of Economics, said monarchs such as Elizabeth II and Victoria also went through periods of unpopularity.
While the royal household will be concerned by a fall in public support, there has been a general decline in support for British political institutions and there is a lack of consensus about what the monarchy would be replaced with, he said.
"Making radical change to how Britain's political system works is always really quite difficult," he said.
Charles wants a slimmed-down monarchy which would be less expensive to run and his mother said the royal family only existed with the support of the people.
While the majority of the British newspapers still support the royal family, covering their tours of factories or schools, the few which are more sceptical, such as the Guardian, have examined the opaque finances of the family. It has put the king's personal wealth at almost 2 billion pounds ($2.5 billion).
Demonstrations against the monarchy are also planned in the capitals of Scotland and Wales on the day of the coronation.
Scotland's new leader, Humza Yousaf, elected last month, said he wanted to end the monarchy. Wales's leader Mark Drakeford also wants a republic, meaning two of the four nations that make up the United Kingdom are led by republicans.
Republic's Smith predicts support for the royal family will continue to decline, and a future government will eventually decide to hold a referendum on the issue.
"Younger people are moving away from the royal family in their droves," he said. "The queen was the monarchy for so many people. They were willing to suspend disbelief and to suspend judgment. Now she is not there, she is no longer there to protect the monarchy."